Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Paolo Bonzini]

On 15/09/2016 17:23, Alex Bligh wrote:
> Paolo,
> 
>> On 15 Sep 2016, at 15:07, Paolo Bonzini  wrote:
>>
>> I don't think QEMU forbids multiple clients to the single server, and
>> guarantees consistency as long as there is no overlap between writes and
>> reads.  These are the same guarantees you have for multiple commands on
>> a single connection.
>>
>> In other words, from the POV of QEMU there's no difference whether
>> multiple commands come from one or more connections.
> 
> This isn't really about ordering, it's about cache coherency
> and persisting things to disk.
> 
> What you say is correct as far as it goes in terms of ordering. However
> consider the scenario with read and writes on two channels as follows
> of the same block:
> 
>  Channel1 Channel2
> 
>  R  Block read, and cached in user space in
> channel 1's cache
> Reply sent
> 
>   W New value written, channel 2's cache updated
> channel 1's cache not
> 
>  R  Value returned from channel 1's cache.
> 
> 
> In the above scenario, there is a problem if the server(s) handling the
> two channels each use a read cache which is not coherent between the
> two channels. An example would be a read-through cache on a server that
> did fork() and shared no state between connections.

qemu-nbd does not fork(), so there is no coherency issue if W has replied.

However, if W hasn't replied, channel1 can get garbage.  Typically the
VM will be the one during writes, everyone else must be ready to handle
whatever mess the VM throws at them.

Paolo

> Similarly, if there is a write on channel 1 that has completed, and
> the flush goes to channel 2, it may not (if state is not shared) guarantee
> that the write on channel 1 (which has completed) is persisted to non-volatile
> media. Obviously if the 'state' is OS block cache/buffers/whatever, it
> will, but if it's (e.g.) a user-space per process write-through cache,
> it won't.
> 
> I don't know whether qemu-nbd is likely to suffer from either of these.

It can't happen.  On the other hand, channel1 must be ready to handle
garbage, it's illegal.






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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Eric Blake]

On 09/15/2016 11:27 AM, Wouter Verhelst wrote:
> On Thu, Sep 15, 2016 at 05:08:21PM +0100, Alex Bligh wrote:
>> Wouter,
>>
>>> The server can always refuse to allow multiple connections.
>>
>> Sure, but it would be neater to warn the client of that at negotiation
>> stage (it would only be one flag, e.g.  'multiple connections
>> unsafe').
> 
> I suppose that's not a bad idea.

Seems like it may need to be a per-export flag, rather than a global
flag (as a given server may be able to serve multiple types of files,
where the safety depends on the type of file being served).


-- 
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Libvirt virtualization library http://libvirt.org



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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

Wouter,

> On 15 Sep 2016, at 17:27, Wouter Verhelst  wrote:
> 
> On Thu, Sep 15, 2016 at 05:08:21PM +0100, Alex Bligh wrote:
>> Wouter,
>> 
>>> The server can always refuse to allow multiple connections.
>> 
>> Sure, but it would be neater to warn the client of that at negotiation
>> stage (it would only be one flag, e.g.  'multiple connections
>> unsafe').
> 
> I suppose that's not a bad idea.

Good.

> [...]
>>> I was thinking of changing the spec as follows:
>>> 
>>> diff --git a/doc/proto.md b/doc/proto.md
>>> index 217f57e..cb099e2 100644
>>> --- a/doc/proto.md
>>> +++ b/doc/proto.md
>>> @@ -308,6 +308,23 @@ specification, the
>>> [kernel 
>>> documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt)
>>> may be useful.
>>> 
>>> +For performance reasons, clients MAY open multiple connections to the
>>> +same server. To support such clients, servers SHOULD ensure that at
>>> +least one of the following conditions hold:
>>> +
>>> +* Flush commands are processed for ALL connections. That is, when an
>>> +  `NBD_CMD_WRITE` is processed on one connection, and then an
>>> +  `NBD_CMD_FLUSH` is processed on another connection, the data of the
>>> +  `NBD_CMD_WRITE` on the first connection MUST reach permanent storage
>>> +  before the reply of the `NBD_CMD_FLUSH` is sent.
>>> +* The server allows `NBD_CMD_WRITE` and `NBD_CMD_FLUSH` on at most one
>>> +  connection
>>> +* Multiple connections are not allowed
>>> +
>>> +In addition, clients using multiple connections SHOULD NOT send
>>> +`NBD_CMD_FLUSH` if an `NBD_CMD_WRITE` for which they care in relation to
>>> +the flush has not been replied to yet.
>>> +
>> 
>> I don't think that should be a mandatory behaviour.
> 
> Which part of it?

I've read it again :-) The wording was slightly contorted. I think
what I mean is that if you don't support flush at all, that's
another option.

The final paragraph I am not sure is right, as that's not what the kernel
currently does. If we are going to suggest a change in our main client's
behaviour, should we not just request that flush is done on all channels?

>> For once, it would
>> be reasonably easy on gonbdserver but a PITA on the reference server.
>> You'd need to put in IPC between each of the forked processes OR rely
>> on fdatasync() only - I'm not sure that would necessarily work
>> safely with (e.g.) the 'treefiles' / COW options.
>> 
>> I think better would be to say that the server MUST either
>> 
>> * Not support NBD_CMD_FLUSH at all
> 
> I think we should discourage not supporting FLUSH, rather than
> suggesting it. 

Sure, but some backends just don't support flush. For them, this
aspect at least is not a problem.

>> * Support NBD_CMD_FLUSH across channels (as you set out above), or
>> * Indicate that it does not support multiple channels.
> 
> You dropped the one with no writes. I said "at most" there for a reason.
> Originally I was going to say "if the server is read-only", but then
> thought that it could work to do the "at most" thing. After having given
> that some more thought, I now realize that if you write, you need to
> flush across to other channels, regardless of whether they write too, so
> that bit of it is moot now anyway.
> 
> Still, a server which exports read-only should still be safe for
> multiple connections, even if there is no cache coherency (since
> presumably nothing's going to change anyway).

Yes

>> Actually I think this is a problem anyway. A simpler failure case is
>> one where (by chance) one channel gets the writes, and one channel
>> gets the flushes. The flush reply is delayed beyond the replies to
>> unconnected writes (on the other channel) and hence the kernel thinks
>> replied-to writes have been persisted when they have not been.
> 
> Yes, that is another example of essentially the same problem.

Yeah, I was just trying to simplify things.

>> The only way to fix that (as far as I can see) without changing flush
>> semantics is for the block layer to issue flush requests on each
>> channel when flushing on one channel.
> 
> Christoph just said that that doesn't (currently) happen; I don't know
> whether the kernel currently already (optimistically) sends out flush
> requests before the writes that it expects to hit permanent storage have
> finished, but if it doesn't do that, then there is no problem and my
> suggested bit of spec would be okay.
> 
> If there are good reasons to do so, however, we do indeed have a problem
> and something else is necessary. I don't think flushing across all
> connections is the best solution, though.

Well, the way I look at it is that we have a proposed change in
client behaviour (multiple channels) which causes problems at
least with flush and also (I think) with cache coherency (see other
email). We should either not make that change, or ensure other changes
are added which mitigate these issues.

Flush is actually the obvious one. Cache coherency is far more
subtle (though 

Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

On Thu, Sep 15, 2016 at 05:08:21PM +0100, Alex Bligh wrote:
> Wouter,
> 
> > The server can always refuse to allow multiple connections.
> 
> Sure, but it would be neater to warn the client of that at negotiation
> stage (it would only be one flag, e.g.  'multiple connections
> unsafe').

I suppose that's not a bad idea.

[...]
> > I was thinking of changing the spec as follows:
> > 
> > diff --git a/doc/proto.md b/doc/proto.md
> > index 217f57e..cb099e2 100644
> > --- a/doc/proto.md
> > +++ b/doc/proto.md
> > @@ -308,6 +308,23 @@ specification, the
> > [kernel 
> > documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt)
> > may be useful.
> > 
> > +For performance reasons, clients MAY open multiple connections to the
> > +same server. To support such clients, servers SHOULD ensure that at
> > +least one of the following conditions hold:
> > +
> > +* Flush commands are processed for ALL connections. That is, when an
> > +  `NBD_CMD_WRITE` is processed on one connection, and then an
> > +  `NBD_CMD_FLUSH` is processed on another connection, the data of the
> > +  `NBD_CMD_WRITE` on the first connection MUST reach permanent storage
> > +  before the reply of the `NBD_CMD_FLUSH` is sent.
> > +* The server allows `NBD_CMD_WRITE` and `NBD_CMD_FLUSH` on at most one
> > +  connection
> > +* Multiple connections are not allowed
> > +
> > +In addition, clients using multiple connections SHOULD NOT send
> > +`NBD_CMD_FLUSH` if an `NBD_CMD_WRITE` for which they care in relation to
> > +the flush has not been replied to yet.
> > +
> 
> I don't think that should be a mandatory behaviour.

Which part of it?

> For once, it would
> be reasonably easy on gonbdserver but a PITA on the reference server.
> You'd need to put in IPC between each of the forked processes OR rely
> on fdatasync() only - I'm not sure that would necessarily work
> safely with (e.g.) the 'treefiles' / COW options.
> 
> I think better would be to say that the server MUST either
> 
> * Not support NBD_CMD_FLUSH at all

I think we should discourage not supporting FLUSH, rather than
suggesting it. 

> * Support NBD_CMD_FLUSH across channels (as you set out above), or
> * Indicate that it does not support multiple channels.

You dropped the one with no writes. I said "at most" there for a reason.
Originally I was going to say "if the server is read-only", but then
thought that it could work to do the "at most" thing. After having given
that some more thought, I now realize that if you write, you need to
flush across to other channels, regardless of whether they write too, so
that bit of it is moot now anyway.

Still, a server which exports read-only should still be safe for
multiple connections, even if there is no cache coherency (since
presumably nothing's going to change anyway).

[...]
> > The latter bit (on the client side) is because even if your backend has
> > no cache coherency issues, TCP does not guarantee ordering between
> > multiple connections. I don't know if the above is in line with what
> > blk-mq does, but consider the following scenario:
> > 
> > - A client sends two writes to the server, followed (immediately) by a
> >  flush, where at least the second write and the flush are not sent over
> >  the same connection.
> > - The first write is a small one, and it is handled almost immediately.
> > - The second write takes a little longer, so the flush is handled
> >  earlier than the second write
> > - The network packet containing the flush reply gets lost for whatever
> >  reason, so the client doesn't get it, and we fall into TCP
> >  retransmits.
> > - The second write finishes, and its reply header does not get lost
> > - After the second write reply reaches the client, the TCP retransmits
> >  for the flush reply are handled.
> > 
> > In the above scenario, the flush reply arrives on the client side after
> > a write reply which it did not cover; so the client will (incorrectly)
> > assume that the write has reached permanent storage when in fact it may
> > not have done so yet.
> > 
> > If the kernel does not care about the ordering of the two writes versus
> > the flush, then there is no problem. I don't know how blk-mq works in
> > that context, but if the above is a likely scenario, we may have to
> > reconsider adding blk-mq to nbd.
> 
> Actually I think this is a problem anyway. A simpler failure case is
> one where (by chance) one channel gets the writes, and one channel
> gets the flushes. The flush reply is delayed beyond the replies to
> unconnected writes (on the other channel) and hence the kernel thinks
> replied-to writes have been persisted when they have not been.

Yes, that is another example of essentially the same problem.

> The only way to fix that (as far as I can see) without changing flush
> semantics is for the block layer to issue flush requests on each
> channel when flushing on one channel.

Christoph just said that that doesn't (currently) happen; I don't know

Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

Wouter,

> The server can always refuse to allow multiple connections.

Sure, but it would be neater to warn the client of that
at negotiation stage (it would only be one flag, e.g.
'multiple connections unsafe'). That way the connection
won't fail with a cryptic EBUSY or whatever, but will
just negotiate a single connection.

> I was thinking of changing the spec as follows:
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 217f57e..cb099e2 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -308,6 +308,23 @@ specification, the
> [kernel 
> documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt)
> may be useful.
> 
> +For performance reasons, clients MAY open multiple connections to the
> +same server. To support such clients, servers SHOULD ensure that at
> +least one of the following conditions hold:
> +
> +* Flush commands are processed for ALL connections. That is, when an
> +  `NBD_CMD_WRITE` is processed on one connection, and then an
> +  `NBD_CMD_FLUSH` is processed on another connection, the data of the
> +  `NBD_CMD_WRITE` on the first connection MUST reach permanent storage
> +  before the reply of the `NBD_CMD_FLUSH` is sent.
> +* The server allows `NBD_CMD_WRITE` and `NBD_CMD_FLUSH` on at most one
> +  connection
> +* Multiple connections are not allowed
> +
> +In addition, clients using multiple connections SHOULD NOT send
> +`NBD_CMD_FLUSH` if an `NBD_CMD_WRITE` for which they care in relation to
> +the flush has not been replied to yet.
> +

I don't think that should be a mandatory behaviour. For once, it would
be reasonably easy on gonbdserver but a PITA on the reference server.
You'd need to put in IPC between each of the forked processes OR rely
on fdatasync() only - I'm not sure that would necessarily work
safely with (e.g.) the 'treefiles' / COW options.

I think better would be to say that the server MUST either

* Not support NBD_CMD_FLUSH at all
* Support NBD_CMD_FLUSH across channels (as you set out above), or
* Indicate that it does not support multiple channels.

>  Request message
> 
> The request message, sent by the client, looks as follows:
> 
> The latter bit (on the client side) is because even if your backend has
> no cache coherency issues, TCP does not guarantee ordering between
> multiple connections. I don't know if the above is in line with what
> blk-mq does, but consider the following scenario:
> 
> - A client sends two writes to the server, followed (immediately) by a
>  flush, where at least the second write and the flush are not sent over
>  the same connection.
> - The first write is a small one, and it is handled almost immediately.
> - The second write takes a little longer, so the flush is handled
>  earlier than the second write
> - The network packet containing the flush reply gets lost for whatever
>  reason, so the client doesn't get it, and we fall into TCP
>  retransmits.
> - The second write finishes, and its reply header does not get lost
> - After the second write reply reaches the client, the TCP retransmits
>  for the flush reply are handled.
> 
> In the above scenario, the flush reply arrives on the client side after
> a write reply which it did not cover; so the client will (incorrectly)
> assume that the write has reached permanent storage when in fact it may
> not have done so yet.
> 
> If the kernel does not care about the ordering of the two writes versus
> the flush, then there is no problem. I don't know how blk-mq works in
> that context, but if the above is a likely scenario, we may have to
> reconsider adding blk-mq to nbd.

Actually I think this is a problem anyway. A simpler failure case is
one where (by chance) one channel gets the writes, and one channel
gets the flushes. The flush reply is delayed beyond the replies to
unconnected writes (on the other channel) and hence the kernel thinks
replied-to writes have been persisted when they have not been.

The only way to fix that (as far as I can see) without changing flush
semantics is for the block layer to issue flush requests on each
channel when flushing on one channel. This, incidentally, would
resolve every other issue!

-- 
Alex Bligh





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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

Eric,

> I doubt that qemu-nbd would ever want to support the situation with more
> than one client connection writing to the same image at the same time;
> the implications of sorting out data consistency between multiple
> writers is rather complex and not worth coding into qemu.  So I think
> qemu would probably prefer to just prohibit the multiple writer
> situation.

Yeah, I was thinking about a 'no multiple connection' flag.

>  And while multiple readers with no writer should be fine,
> I'm not even sure if multiple readers plus one writer can always be made
> to appear sane (if there is no coordination between the different
> connections, on an image where the writer changes AA to BA then flushes
> then changes to BB, it is still feasible that a reader could see AB
> (pre-flush state of the first sector, post-flush changes to the second
> sector, even though the writer never flushed that particular content to
> disk).

Agree

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

Paolo,

> On 15 Sep 2016, at 15:07, Paolo Bonzini  wrote:
> 
> I don't think QEMU forbids multiple clients to the single server, and
> guarantees consistency as long as there is no overlap between writes and
> reads.  These are the same guarantees you have for multiple commands on
> a single connection.
> 
> In other words, from the POV of QEMU there's no difference whether
> multiple commands come from one or more connections.

This isn't really about ordering, it's about cache coherency
and persisting things to disk.

What you say is correct as far as it goes in terms of ordering. However
consider the scenario with read and writes on two channels as follows
of the same block:

 Channel1 Channel2

 R  Block read, and cached in user space in
channel 1's cache
Reply sent

  W New value written, channel 2's cache updated
channel 1's cache not

 R  Value returned from channel 1's cache.


In the above scenario, there is a problem if the server(s) handling the
two channels each use a read cache which is not coherent between the
two channels. An example would be a read-through cache on a server that
did fork() and shared no state between connections.

Similarly, if there is a write on channel 1 that has completed, and
the flush goes to channel 2, it may not (if state is not shared) guarantee
that the write on channel 1 (which has completed) is persisted to non-volatile
media. Obviously if the 'state' is OS block cache/buffers/whatever, it
will, but if it's (e.g.) a user-space per process write-through cache,
it won't.

I don't know whether qemu-nbd is likely to suffer from either of these.

--
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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Paolo Bonzini]

On 15/09/2016 15:34, Eric Blake wrote:
> On 09/15/2016 06:09 AM, Alex Bligh wrote:
>>
>> I also wonder whether any servers that can do caching per
>> connection will always share a consistent cache between 
>> connections. The one I'm worried about in particular here
>> is qemu-nbd - Eric Blake CC'd.
>>
> 
> I doubt that qemu-nbd would ever want to support the situation with more
> than one client connection writing to the same image at the same time;
> the implications of sorting out data consistency between multiple
> writers is rather complex and not worth coding into qemu.  So I think
> qemu would probably prefer to just prohibit the multiple writer
> situation.  And while multiple readers with no writer should be fine,
> I'm not even sure if multiple readers plus one writer can always be made
> to appear sane (if there is no coordination between the different
> connections, on an image where the writer changes AA to BA then flushes
> then changes to BB, it is still feasible that a reader could see AB
> (pre-flush state of the first sector, post-flush changes to the second
> sector, even though the writer never flushed that particular content to
> disk).
> 
> But Paolo Bonzini (cc'd) may have more insight on qemu's NBD server and
> what it supports (or forbids) in the way of multiple clients to a single
> server.

I don't think QEMU forbids multiple clients to the single server, and
guarantees consistency as long as there is no overlap between writes and
reads.  These are the same guarantees you have for multiple commands on
a single connection.

In other words, from the POV of QEMU there's no difference whether
multiple commands come from one or more connections.

Paolo

>> A more general point is that with multiple queues requests
>> may be processed in a different order even by those servers that
>> currently process the requests in strict order, or in something
>> similar to strict order. The server is permitted by the spec
>> (save as mandated by NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA) to
>> process commands out of order anyway, but I suspect this has
>> to date been little tested.
> 
> qemu-nbd is definitely capable of serving reads and writes out-of-order
> to a single connection client; but that's different than the case with
> multiple connections.
> 



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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Josef Bacik]

On 09/15/2016 09:17 AM, Wouter Verhelst wrote:
> On Thu, Sep 15, 2016 at 01:44:29PM +0100, Alex Bligh wrote:
>>
>>> On 15 Sep 2016, at 13:41, Christoph Hellwig  wrote:
>>>
>>> On Thu, Sep 15, 2016 at 01:39:11PM +0100, Alex Bligh wrote:
 That's probably right in the case of file-based back ends that
 are running on a Linux OS. But gonbdserver for instance supports
 (e.g.) Ceph based backends, where each connection might be talking
 to a completely separate ceph node, and there may be no cache
 consistency between connections.
>>>
>>> Yes, if you don't have a cache coherent backend you are generally
>>> screwed with a multiqueue protocol.
>>
>> I wonder if the ability to support multiqueue should be visible
>> in the negotiation stage. That would allow the client to refuse
>> to select multiqueue where it isn't safe.
>
> The server can always refuse to allow multiple connections.
>
> I was thinking of changing the spec as follows:
>
> diff --git a/doc/proto.md b/doc/proto.md
> index 217f57e..cb099e2 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -308,6 +308,23 @@ specification, the
>  [kernel 
> documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt)
>  may be useful.
>
> +For performance reasons, clients MAY open multiple connections to the
> +same server. To support such clients, servers SHOULD ensure that at
> +least one of the following conditions hold:
> +
> +* Flush commands are processed for ALL connections. That is, when an
> +  `NBD_CMD_WRITE` is processed on one connection, and then an
> +  `NBD_CMD_FLUSH` is processed on another connection, the data of the
> +  `NBD_CMD_WRITE` on the first connection MUST reach permanent storage
> +  before the reply of the `NBD_CMD_FLUSH` is sent.
> +* The server allows `NBD_CMD_WRITE` and `NBD_CMD_FLUSH` on at most one
> +  connection
> +* Multiple connections are not allowed
> +
> +In addition, clients using multiple connections SHOULD NOT send
> +`NBD_CMD_FLUSH` if an `NBD_CMD_WRITE` for which they care in relation to
> +the flush has not been replied to yet.
> +
>   Request message
>
>  The request message, sent by the client, looks as follows:
>
> The latter bit (on the client side) is because even if your backend has
> no cache coherency issues, TCP does not guarantee ordering between
> multiple connections. I don't know if the above is in line with what
> blk-mq does, but consider the following scenario:
>
> - A client sends two writes to the server, followed (immediately) by a
>   flush, where at least the second write and the flush are not sent over
>   the same connection.
> - The first write is a small one, and it is handled almost immediately.
> - The second write takes a little longer, so the flush is handled
>   earlier than the second write
> - The network packet containing the flush reply gets lost for whatever
>   reason, so the client doesn't get it, and we fall into TCP
>   retransmits.
> - The second write finishes, and its reply header does not get lost
> - After the second write reply reaches the client, the TCP retransmits
>   for the flush reply are handled.
>
> In the above scenario, the flush reply arrives on the client side after
> a write reply which it did not cover; so the client will (incorrectly)
> assume that the write has reached permanent storage when in fact it may
> not have done so yet.

This isn't an NBD problem, this is an application problem.  The application 
must 
wait for all writes it cares about _before_ issuing a flush.  This is the same 
as for normal storage as it is for NBD.  It is not NBD's responsibility to 
maintain coherency between multiple requests across connections, just simply to 
act on and respond to requests.

I think changing the specification to indicate that this is the case for 
multiple connections is a good thing, to keep NBD servers from doing weird 
things like sending different connections to the same export to different 
backing stores without some sort of synchronization.  It should definitely be 
explicitly stated somewhere that NBD does not provide any ordering guarantees 
and that is up to the application.  Thanks,

Josef

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Eric Blake]

On 09/15/2016 06:09 AM, Alex Bligh wrote:
> 
> I also wonder whether any servers that can do caching per
> connection will always share a consistent cache between 
> connections. The one I'm worried about in particular here
> is qemu-nbd - Eric Blake CC'd.
> 

I doubt that qemu-nbd would ever want to support the situation with more
than one client connection writing to the same image at the same time;
the implications of sorting out data consistency between multiple
writers is rather complex and not worth coding into qemu.  So I think
qemu would probably prefer to just prohibit the multiple writer
situation.  And while multiple readers with no writer should be fine,
I'm not even sure if multiple readers plus one writer can always be made
to appear sane (if there is no coordination between the different
connections, on an image where the writer changes AA to BA then flushes
then changes to BB, it is still feasible that a reader could see AB
(pre-flush state of the first sector, post-flush changes to the second
sector, even though the writer never flushed that particular content to
disk).

But Paolo Bonzini (cc'd) may have more insight on qemu's NBD server and
what it supports (or forbids) in the way of multiple clients to a single
server.

> A more general point is that with multiple queues requests
> may be processed in a different order even by those servers that
> currently process the requests in strict order, or in something
> similar to strict order. The server is permitted by the spec
> (save as mandated by NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA) to
> process commands out of order anyway, but I suspect this has
> to date been little tested.

qemu-nbd is definitely capable of serving reads and writes out-of-order
to a single connection client; but that's different than the case with
multiple connections.

-- 
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Libvirt virtualization library http://libvirt.org



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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

On Thu, Sep 15, 2016 at 01:44:29PM +0100, Alex Bligh wrote:
> 
> > On 15 Sep 2016, at 13:41, Christoph Hellwig  wrote:
> > 
> > On Thu, Sep 15, 2016 at 01:39:11PM +0100, Alex Bligh wrote:
> >> That's probably right in the case of file-based back ends that
> >> are running on a Linux OS. But gonbdserver for instance supports
> >> (e.g.) Ceph based backends, where each connection might be talking
> >> to a completely separate ceph node, and there may be no cache
> >> consistency between connections.
> > 
> > Yes, if you don't have a cache coherent backend you are generally
> > screwed with a multiqueue protocol.
> 
> I wonder if the ability to support multiqueue should be visible
> in the negotiation stage. That would allow the client to refuse
> to select multiqueue where it isn't safe.

The server can always refuse to allow multiple connections.

I was thinking of changing the spec as follows:

diff --git a/doc/proto.md b/doc/proto.md
index 217f57e..cb099e2 100644
--- a/doc/proto.md
+++ b/doc/proto.md
@@ -308,6 +308,23 @@ specification, the
 [kernel 
documentation](https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt)
 may be useful.
 
+For performance reasons, clients MAY open multiple connections to the
+same server. To support such clients, servers SHOULD ensure that at
+least one of the following conditions hold:
+
+* Flush commands are processed for ALL connections. That is, when an
+  `NBD_CMD_WRITE` is processed on one connection, and then an
+  `NBD_CMD_FLUSH` is processed on another connection, the data of the
+  `NBD_CMD_WRITE` on the first connection MUST reach permanent storage
+  before the reply of the `NBD_CMD_FLUSH` is sent.
+* The server allows `NBD_CMD_WRITE` and `NBD_CMD_FLUSH` on at most one
+  connection
+* Multiple connections are not allowed
+
+In addition, clients using multiple connections SHOULD NOT send
+`NBD_CMD_FLUSH` if an `NBD_CMD_WRITE` for which they care in relation to
+the flush has not been replied to yet.
+
  Request message
 
 The request message, sent by the client, looks as follows:

The latter bit (on the client side) is because even if your backend has
no cache coherency issues, TCP does not guarantee ordering between
multiple connections. I don't know if the above is in line with what
blk-mq does, but consider the following scenario:

- A client sends two writes to the server, followed (immediately) by a
  flush, where at least the second write and the flush are not sent over
  the same connection.
- The first write is a small one, and it is handled almost immediately.
- The second write takes a little longer, so the flush is handled
  earlier than the second write
- The network packet containing the flush reply gets lost for whatever
  reason, so the client doesn't get it, and we fall into TCP
  retransmits.
- The second write finishes, and its reply header does not get lost
- After the second write reply reaches the client, the TCP retransmits
  for the flush reply are handled.

In the above scenario, the flush reply arrives on the client side after
a write reply which it did not cover; so the client will (incorrectly)
assume that the write has reached permanent storage when in fact it may
not have done so yet.

If the kernel does not care about the ordering of the two writes versus
the flush, then there is no problem. I don't know how blk-mq works in
that context, but if the above is a likely scenario, we may have to
reconsider adding blk-mq to nbd.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
   people in the world who think they really understand all of its rules,
   and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

> On 15 Sep 2016, at 13:41, Christoph Hellwig  wrote:
> 
> On Thu, Sep 15, 2016 at 01:39:11PM +0100, Alex Bligh wrote:
>> That's probably right in the case of file-based back ends that
>> are running on a Linux OS. But gonbdserver for instance supports
>> (e.g.) Ceph based backends, where each connection might be talking
>> to a completely separate ceph node, and there may be no cache
>> consistency between connections.
> 
> Yes, if you don't have a cache coherent backend you are generally
> screwed with a multiqueue protocol.

I wonder if the ability to support multiqueue should be visible
in the negotiation stage. That would allow the client to refuse
to select multiqueue where it isn't safe.

Wouter?

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

> On 15 Sep 2016, at 13:23, Christoph Hellwig  wrote:
> 
> On Thu, Sep 15, 2016 at 02:21:20PM +0200, Wouter Verhelst wrote:
>> Right. So do I understand you correctly that blk-mq currently doesn't
>> look at multiple queues, and just assumes that if a FLUSH is sent over
>> any one of the queues, it applies to all queues?
> 
> Yes.  The same is true at the protocol level for NVMe or SCSI transports
> that can make use of multiple queues.

At an implementation level that is going to be a little difficult
for some NBD servers, e.g. ones that fork() a different process per
connection. There is in general no IPC to speak of between server
instances. Such servers would thus be unsafe with more than one
connection if FLUSH is in use.

I believe such servers include the reference server where there is
process per connection (albeit possibly with several threads).

Even single process servers (including mine - gonbdserver) would
require logic to pair up multiple connections to the same
device.

I suspect ALL nbd servers would require a change. So I think we should
think carefully before introducing this protocol change.

It might be easier to make the client code change work around this
issue (somehow).

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 01:39:11PM +0100, Alex Bligh wrote:
> That's probably right in the case of file-based back ends that
> are running on a Linux OS. But gonbdserver for instance supports
> (e.g.) Ceph based backends, where each connection might be talking
> to a completely separate ceph node, and there may be no cache
> consistency between connections.

Yes, if you don't have a cache coherent backend you are generally
screwed with a multiqueue protocol.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

> On 15 Sep 2016, at 13:36, Christoph Hellwig  wrote:
> 
> On Thu, Sep 15, 2016 at 01:33:20PM +0100, Alex Bligh wrote:
>> At an implementation level that is going to be a little difficult
>> for some NBD servers, e.g. ones that fork() a different process per
>> connection. There is in general no IPC to speak of between server
>> instances. Such servers would thus be unsafe with more than one
>> connection if FLUSH is in use.
>> 
>> I believe such servers include the reference server where there is
>> process per connection (albeit possibly with several threads).
>> 
>> Even single process servers (including mine - gonbdserver) would
>> require logic to pair up multiple connections to the same
>> device.
> 
> Why?  If you only send the completion after your I/O syscall returned
> your are fine if fsync comes from a difference process, no matter
> if you're using direct or buffered I/O underneath.

That's probably right in the case of file-based back ends that
are running on a Linux OS. But gonbdserver for instance supports
(e.g.) Ceph based backends, where each connection might be talking
to a completely separate ceph node, and there may be no cache
consistency between connections.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 01:33:20PM +0100, Alex Bligh wrote:
> At an implementation level that is going to be a little difficult
> for some NBD servers, e.g. ones that fork() a different process per
> connection. There is in general no IPC to speak of between server
> instances. Such servers would thus be unsafe with more than one
> connection if FLUSH is in use.
> 
> I believe such servers include the reference server where there is
> process per connection (albeit possibly with several threads).
> 
> Even single process servers (including mine - gonbdserver) would
> require logic to pair up multiple connections to the same
> device.

Why?  If you only send the completion after your I/O syscall returned
your are fine if fsync comes from a difference process, no matter
if you're using direct or buffered I/O underneath.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

> On 15 Sep 2016, at 13:18, Christoph Hellwig  wrote:
> 
> Yes, please do that.  A "barrier" implies draining of the queue.

Done

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 02:21:20PM +0200, Wouter Verhelst wrote:
> Right. So do I understand you correctly that blk-mq currently doesn't
> look at multiple queues, and just assumes that if a FLUSH is sent over
> any one of the queues, it applies to all queues?

Yes.  The same is true at the protocol level for NVMe or SCSI transports
that can make use of multiple queues.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 02:01:59PM +0200, Wouter Verhelst wrote:
> Yes. There was some discussion on that part, and we decided that setting
> the flag doesn't hurt, but the spec also clarifies that using it on READ
> does nothing, semantically.
>
> 
> The problem is that there are clients in the wild which do set it on
> READ, so it's just a matter of "be liberal in what you accept".

Note that FUA on READ in SCSI and NVMe does have a meaning - it
requires you to bypass any sort of cache on the target.  I think it's an
wrong defintion because it mandates implementation details that aren't
observable by the initiator, but it's still the spec wording and nbd
diverges from it.  That's not nessecarily a bad thing, but a caveat to
look out for.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 01:11:24PM +0100, Alex Bligh wrote:
> > NBD_CMD_FLUSH (3)
> > 
> > A flush request; a write barrier. 
> 
> I can see that's potentially confusing as isn't meant to mean 'an old-style
> linux kernel block device write barrier'. I think in general terms it
> probably is some form of barrier, but I see no problem in deleting the
> words "a write barrier" from the spec text if only to make it
> clearer.

Yes, please do that.  A "barrier" implies draining of the queue.

> However, I think the description of the command itself:
> 
> > The server MUST NOT send a successful reply header for this request before 
> > all write requests for which a reply has already been sent to the client 
> > have reached permanent storage (using fsync() or similar).
> 
> and the ordering section I pointed you to before, were both correct, yes?

Yes, this seems correct.

> actually fdatasync() technically does more than is necessary, as it
> will also flush commands that have been processed, but for which no
> reply has yet been sent - that's no bad thing.

Yes.  But without an actual barrier it's hard to be exact - and
fdatasync does the right thing by including false positives.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

Christoph,

> It's not a write barrier - a write barrier was command that ensured that
> 
> a) all previous writes were completed to the host/client
> b) all previous writes were on non-volatile storage
> 
> and
> 
> c) the actual write with the barrier bit was on non-volatile storage

Ah! the bit you are complaining about is not the bit I pointed to you, but:

> NBD_CMD_FLUSH (3)
> 
> A flush request; a write barrier. 

I can see that's potentially confusing as isn't meant to mean 'an old-style
linux kernel block device write barrier'. I think in general terms it
probably is some form of barrier, but I see no problem in deleting the
words "a write barrier" from the spec text if only to make it
clearer. However, I think the description of the command itself:

> The server MUST NOT send a successful reply header for this request before 
> all write requests for which a reply has already been sent to the client have 
> reached permanent storage (using fsync() or similar).

and the ordering section I pointed you to before, were both correct, yes?


>> The point still remains that "X was sent before Y" is difficult to
>> determine on the client side if X was sent over a different TCP channel
>> than Y, because a packet might be dropped (requiring a retransmit) for
>> X, and similar things. If blk-mq can deal with that, we're good and
>> nothing further needs to be done. If not, this should be evaluated by
>> someone more familiar with the internals of the kernel block subsystem
>> than me.
> 
> The important bit in all the existing protocols, and which Linux relies
> on is that any write the Linux block layer got a completion for earlier
> needs to be flushed out to non-volatile storage when a FLUSH command is
> set.  Anything that still is in flight does not matter.  Which for
> NBD means anything that you already replies to need to be flushed.

... that's what it says (I hope).

> Or to say it more practicly - in the nbd server you simply need to
> call fdatasync on the backing device or file whenever you get a FLUSH
> requires, and it will do the right thing.

actually fdatasync() technically does more than is necessary, as it
will also flush commands that have been processed, but for which no
reply has yet been sent - that's no bad thing.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

> On 15 Sep 2016, at 12:52, Christoph Hellwig  wrote:
> 
> On Thu, Sep 15, 2016 at 12:46:07PM +0100, Alex Bligh wrote:
>> Essentially NBD does supports FLUSH/FUA like this:
>> 
>> https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt
>> 
>> IE supports the same FLUSH/FUA primitives as other block drivers (AIUI).
>> 
>> Link to protocol (per last email) here:
>> 
>> https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes
> 
> Flush as defined by the Linux block layer (and supported that way in
> SCSI, ATA, NVMe) only requires to flush all already completed writes
> to non-volatile media.  It does not impose any ordering unlike the
> nbd spec.

As maintainer of the NBD spec, I'm confused as to why you think it
imposes any ordering - if you think this, clearly I need to clean up
the wording.

Here's what it says:

> The server MAY process commands out of order, and MAY reply out of order,
> except that:
> 
>   • All write commands (that includes NBD_CMD_WRITE, and NBD_CMD_TRIM)
> that the server completes (i.e. replies to) prior to processing to a
> NBD_CMD_FLUSH MUST be written to non-volatile storage prior to replying to 
> that
> NBD_CMD_FLUSH. This paragraph only applies if NBD_FLAG_SEND_FLUSH is set 
> within
> the transmission flags, as otherwise NBD_CMD_FLUSH will never be sent by the
> client to the server.

(and another bit re FUA that isn't relevant here).

Here's the Linux Kernel documentation:

> The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
> the filesystem and will make sure the volatile cache of the storage device
> has been flushed before the actual I/O operation is started.  This explicitly
> guarantees that previously completed write requests are on non-volatile
> storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can 
> be
> set on an otherwise empty bio structure, which causes only an explicit cache
> flush without any dependent I/O.  It is recommend to use
> the blkdev_issue_flush() helper for a pure cache flush.


I believe that NBD treats NBD_CMD_FLUSH the same as a REQ_PREFLUSH and empty
bio.

If you don't read those two as compatible, I'd like to understand why not
(i.e. what additional constraints one is applying that the other is not)
as they are meant to be the same (save that NBD only has FLUSH as a command,
i.e. the 'empty bio' version). I am happy to improve the docs to make it
clearer.

(sidenote: I am interested in the change from REQ_FLUSH to REQ_PREFLUSH,
but in an empty bio it's not really relevant I think).

> FUA as defined by the Linux block layer (and supported that way in SCSI,
> ATA, NVMe) only requires the write operation the FUA bit is set on to be
> on non-volatile media before completing the write operation.  It does
> not impose any ordering, which seems to match the nbd spec.  Unlike the
> NBD spec Linux does not allow FUA to be set on anything by WRITE
> commands.  Some other storage protocols allow a FUA bit on READ
> commands or other commands that write data to the device, though.

I think you mean "anything *but* WRITE commands". In NBD setting
FUA on a command that does not write will do nothing, but FUA can
be set on NBD_CMD_TRIM and has the expected effect.

Interestingly the kernel docs are silent on which commands REQ_FUA
can be set on.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

On Thu, Sep 15, 2016 at 04:52:17AM -0700, Christoph Hellwig wrote:
> On Thu, Sep 15, 2016 at 12:46:07PM +0100, Alex Bligh wrote:
> > Essentially NBD does supports FLUSH/FUA like this:
> > 
> > https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt
> > 
> > IE supports the same FLUSH/FUA primitives as other block drivers (AIUI).
> > 
> > Link to protocol (per last email) here:
> > 
> > https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes
> 
> Flush as defined by the Linux block layer (and supported that way in
> SCSI, ATA, NVMe) only requires to flush all already completed writes
> to non-volatile media.

That is precisely what FLUSH in nbd does, too.

> It does not impose any ordering unlike the nbd spec.

If you read the spec differently, then that's a bug in the spec. Can you
clarify which part of it caused that confusion? We should fix it, then.

> FUA as defined by the Linux block layer (and supported that way in SCSI,
> ATA, NVMe) only requires the write operation the FUA bit is set on to be
> on non-volatile media before completing the write operation.  It does
> not impose any ordering, which seems to match the nbd spec.  Unlike the
> NBD spec Linux does not allow FUA to be set on anything by WRITE
> commands.  Some other storage protocols allow a FUA bit on READ
> commands or other commands that write data to the device, though.

Yes. There was some discussion on that part, and we decided that setting
the flag doesn't hurt, but the spec also clarifies that using it on READ
does nothing, semantically.

The problem is that there are clients in the wild which do set it on
READ, so it's just a matter of "be liberal in what you accept".

-- 
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   people in the world who think they really understand all of its rules,
   and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 01:55:14PM +0200, Wouter Verhelst wrote:
> Maybe I'm not using the correct terminology here. The point is that
> after a FLUSH, the server asserts that all write commands *for which a
> reply has already been sent to the client* will also have reached
> permanent storage. Nothing is asserted about commands for which the
> reply has not yet been sent, regardless of whether they were sent to the
> server before or after the FLUSH; they may reach permanent storage as a
> result of the FLUSH, or they may not, we don't say.
> 
> With FUA, we only assert that the FUA-flagged command reaches permanent
> storage before its reply is sent, nothing else.

Yes, that's the correct semantics.

> If that's not a write barrier, then I was using the wrong terminology
> (and offer my apologies for the confusion).

It's not a write barrier - a write barrier was command that ensured that

 a) all previous writes were completed to the host/client
 b) all previous writes were on non-volatile storage

and

 c) the actual write with the barrier bit was on non-volatile storage

> The point still remains that "X was sent before Y" is difficult to
> determine on the client side if X was sent over a different TCP channel
> than Y, because a packet might be dropped (requiring a retransmit) for
> X, and similar things. If blk-mq can deal with that, we're good and
> nothing further needs to be done. If not, this should be evaluated by
> someone more familiar with the internals of the kernel block subsystem
> than me.

The important bit in all the existing protocols, and which Linux relies
on is that any write the Linux block layer got a completion for earlier
needs to be flushed out to non-volatile storage when a FLUSH command is
set.  Anything that still is in flight does not matter.  Which for
NBD means anything that you already replies to need to be flushed.

Or to say it more practicly - in the nbd server you simply need to
call fdatasync on the backing device or file whenever you get a FLUSH
requires, and it will do the right thing.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

> On 15 Sep 2016, at 12:46, Christoph Hellwig  wrote:
> 
> On Thu, Sep 15, 2016 at 12:43:35PM +0100, Alex Bligh wrote:
>> Sure, it's at:
>> 
>> https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes
>> 
>> and that link takes you to the specific section.
>> 
>> The treatment of FLUSH and FUA is meant to mirror exactly the
>> linux block layer (or rather how the linux block layer was a few
>> years ago). I even asked on LKML to verify a few points.
> 
> Linux never expected ordering on the wire.  Before 2010 we had barriers
> in the kernel that provided ordering to the caller, but we never
> required it from the protocol / hardware.

Sure. And I think the doc section reflects exactly Linux's post-2010
expectations (note the link to the kernel documentation). IE
servers are not required to order reads/writes (save that all
writes that the server completes prior to reply to an NBD_CMD_FLUSH
must be persisted prior to the reply to that NBD_CMD_FLUSH
which I believe to be exactly the same behaviour as the kernel
dealing with an empty bio with REQ_FLUSH set).

Perhaps the section should be called "No ordering of messages
and writes"!

My point was that *in practice* disordering is not well tested
as *in practice* many server implementations do in fact process
each command in order, though that's changed recently.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

On Thu, Sep 15, 2016 at 04:38:07AM -0700, Christoph Hellwig wrote:
> On Thu, Sep 15, 2016 at 12:49:35PM +0200, Wouter Verhelst wrote:
> > A while back, we spent quite some time defining the semantics of the
> > various commands in the face of the NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA
> > write barriers. At the time, we decided that it would be unreasonable
> > to expect servers to make these write barriers effective across
> > different connections.
> 
> Do you have a nbd protocol specification?

https://github.com/yoe/nbd/blob/master/doc/proto.md

> treating a flush or fua as any sort of barrier is incredibly stupid.

Maybe I'm not using the correct terminology here. The point is that
after a FLUSH, the server asserts that all write commands *for which a
reply has already been sent to the client* will also have reached
permanent storage. Nothing is asserted about commands for which the
reply has not yet been sent, regardless of whether they were sent to the
server before or after the FLUSH; they may reach permanent storage as a
result of the FLUSH, or they may not, we don't say.

With FUA, we only assert that the FUA-flagged command reaches permanent
storage before its reply is sent, nothing else.

If that's not a write barrier, then I was using the wrong terminology
(and offer my apologies for the confusion).

The point still remains that "X was sent before Y" is difficult to
determine on the client side if X was sent over a different TCP channel
than Y, because a packet might be dropped (requiring a retransmit) for
X, and similar things. If blk-mq can deal with that, we're good and
nothing further needs to be done. If not, this should be evaluated by
someone more familiar with the internals of the kernel block subsystem
than me.

> Is it really documented that way, and if yes, why?

The documentation does use the term write barrier, but only right before
the same explanation as above. It does so because I assumed that that is
what a write barrier is, and that this would make things easier to
understand. If you tell me that that term is wrong as used there, I can
easily remove it (it's not critical to the rest of the documentation).

Regards,

-- 
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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 12:46:07PM +0100, Alex Bligh wrote:
> Essentially NBD does supports FLUSH/FUA like this:
> 
> https://www.kernel.org/doc/Documentation/block/writeback_cache_control.txt
> 
> IE supports the same FLUSH/FUA primitives as other block drivers (AIUI).
> 
> Link to protocol (per last email) here:
> 
> https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes

Flush as defined by the Linux block layer (and supported that way in
SCSI, ATA, NVMe) only requires to flush all already completed writes
to non-volatile media.  It does not impose any ordering unlike the
nbd spec.

FUA as defined by the Linux block layer (and supported that way in SCSI,
ATA, NVMe) only requires the write operation the FUA bit is set on to be
on non-volatile media before completing the write operation.  It does
not impose any ordering, which seems to match the nbd spec.  Unlike the
NBD spec Linux does not allow FUA to be set on anything by WRITE
commands.  Some other storage protocols allow a FUA bit on READ
commands or other commands that write data to the device, though.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 12:43:35PM +0100, Alex Bligh wrote:
> Sure, it's at:
> 
> https://github.com/yoe/nbd/blob/master/doc/proto.md#ordering-of-messages-and-writes
> 
> and that link takes you to the specific section.
> 
> The treatment of FLUSH and FUA is meant to mirror exactly the
> linux block layer (or rather how the linux block layer was a few
> years ago). I even asked on LKML to verify a few points.

Linux never expected ordering on the wire.  Before 2010 we had barriers
in the kernel that provided ordering to the caller, but we never
required it from the protocol / hardware.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 12:09:28PM +0100, Alex Bligh wrote:
> A more general point is that with multiple queues requests
> may be processed in a different order even by those servers that
> currently process the requests in strict order, or in something
> similar to strict order. The server is permitted by the spec
> (save as mandated by NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA) to
> process commands out of order anyway, but I suspect this has
> to date been little tested.

The Linux kernel does not assume any synchroniztion between block I/O
commands.  So any sort of synchronization a protocol does is complete
overkill for us.

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Christoph Hellwig]

On Thu, Sep 15, 2016 at 12:49:35PM +0200, Wouter Verhelst wrote:
> A while back, we spent quite some time defining the semantics of the
> various commands in the face of the NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA
> write barriers. At the time, we decided that it would be unreasonable
> to expect servers to make these write barriers effective across
> different connections.

Do you have a nbd protocol specification?  treating a flush or fua
as any sort of barrier is incredibly stupid.  Is it really documented
that way, and if yes, why?

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

On Thu, Sep 15, 2016 at 12:09:28PM +0100, Alex Bligh wrote:
> Wouter, Josef, (& Eric)
> 
> > On 15 Sep 2016, at 11:49, Wouter Verhelst  wrote:
> > 
> > Hi,
> > 
> > On Fri, Sep 09, 2016 at 10:02:03PM +0200, Wouter Verhelst wrote:
> >> I see some practical problems with this:
> > [...]
> > 
> > One more that I didn't think about earlier:
> > 
> > A while back, we spent quite some time defining the semantics of the
> > various commands in the face of the NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA
> > write barriers. At the time, we decided that it would be unreasonable
> > to expect servers to make these write barriers effective across
> > different connections.
> 
> Actually I wonder if there is a wider problem in that implementations
> might mediate access to a device by presence of an extant TCP connection,
> i.e. only permit one TCP connection to access a given block device at
> once. If you think about (for instance) a forking daemon that does
> writeback caching, that would be an entirely reasonable thing to do
> for data consistency.

Sure. They will have to live with the fact that clients connected to
them will run slower; I don't think that's a problem. In addition,
Josef's client implementation requires the user to explicitly ask for
multiple connections.

There are multiple contexts in which NBD can be used, and in some
performance is more important than in others. I think that is fine.

[...]
> A more general point is that with multiple queues requests
> may be processed in a different order even by those servers that
> currently process the requests in strict order, or in something
> similar to strict order. The server is permitted by the spec
> (save as mandated by NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA) to
> process commands out of order anyway, but I suspect this has
> to date been little tested.

Yes, and that is why I was asking about this. If the write barriers
are expected to be shared across connections, we have a problem. If,
however, they are not, then it doesn't matter that the commands may be
processed out of order.

[...]

-- 
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   people in the world who think they really understand all of its rules,
   and pretty much all of them are just lying to themselves too.
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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Alex Bligh]

Wouter, Josef, (& Eric)

> On 15 Sep 2016, at 11:49, Wouter Verhelst  wrote:
> 
> Hi,
> 
> On Fri, Sep 09, 2016 at 10:02:03PM +0200, Wouter Verhelst wrote:
>> I see some practical problems with this:
> [...]
> 
> One more that I didn't think about earlier:
> 
> A while back, we spent quite some time defining the semantics of the
> various commands in the face of the NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA
> write barriers. At the time, we decided that it would be unreasonable
> to expect servers to make these write barriers effective across
> different connections.

Actually I wonder if there is a wider problem in that implementations
might mediate access to a device by presence of an extant TCP connection,
i.e. only permit one TCP connection to access a given block device at
once. If you think about (for instance) a forking daemon that does
writeback caching, that would be an entirely reasonable thing to do
for data consistency.

I also wonder whether any servers that can do caching per
connection will always share a consistent cache between 
connections. The one I'm worried about in particular here
is qemu-nbd - Eric Blake CC'd.

A more general point is that with multiple queues requests
may be processed in a different order even by those servers that
currently process the requests in strict order, or in something
similar to strict order. The server is permitted by the spec
(save as mandated by NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA) to
process commands out of order anyway, but I suspect this has
to date been little tested.

Lastly I confess to lack of familiarity with the kernel side
code, but how is NBD_CMD_DISCONNECT synchronised across
each of the connections? Presumably you need to send it on
each channel, but cannot assume the NBD connection as a whole
is dead until the last tcp connection has closed?

-- 
Alex Bligh





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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

Hi,

On Fri, Sep 09, 2016 at 10:02:03PM +0200, Wouter Verhelst wrote:
> I see some practical problems with this:
[...]

One more that I didn't think about earlier:

A while back, we spent quite some time defining the semantics of the
various commands in the face of the NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA
write barriers. At the time, we decided that it would be unreasonable
to expect servers to make these write barriers effective across
different connections.

Since my knowledge of kernel internals is limited, I tried finding some
documentation on this, but I guess that either it doesn't exist or I'm
looking in the wrong place; therefore, am I correct in assuming that
blk-mq knows about such semantics, and will handle them correctly (by
either sending a write barrier to all queues, or not making assumptions
about write barriers that were sent over a different queue)? If not,
this may be something that needs to be taken care of.

Thanks,

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
   people in the world who think they really understand all of its rules,
   and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Jens Axboe]

On 09/09/2016 05:00 PM, Josef Bacik wrote:
>> Right. Alternatively, you could perhaps make it so that the lost
>> connection is removed, unack'd requests on that connection are resent,
>> and the session moves on with one less connection (unless the lost
>> connection is the last one, in which case we die as before). That might
>> be too much work and not worth it though.
>
> Yeah I wasn't sure if we could just randomly remove hw queue's in blk mq
> while the device is still up.  If that is in fact easy to do then I'm in
> favor of trucking along with less connections than we originally had,
> otherwise I think it'll be too big of a pain.  Also some users (Facebook
> in this case) would rather the whole thing fail so we can figure out
> what went wrong rather than suddenly going at a degraded performance.

We can do that online. We do that for CPU hotplug/unplug events, and we
also expose that functionality to drivers through
blk_mq_update_nr_hw_queues(). So yes, should be trivial to support from
nbd.

-- 
Jens Axboe


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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Josef Bacik]

On 09/09/2016 04:55 PM, Wouter Verhelst wrote:
> On Fri, Sep 09, 2016 at 04:36:07PM -0400, Josef Bacik wrote:
>> On 09/09/2016 04:02 PM, Wouter Verhelst wrote:
> [...]
>>> I see some practical problems with this:
>>> - You removed the pid attribute from sysfs (unless you added it back and
>>>   I didn't notice, in which case just ignore this part). This kills
>>>   userspace in two ways:
>>>   - systemd/udev mark an NBD device as "not active" if the sysfs pid
>>> attribute is absent. Removing that attribute causes the new nbd
>>> systemd unit to stop working.
>>>   - nbd-client -check relies on this attribute too, which means that
>>> even if people don't use systemd, their init scripts will still
>>> break, and vigilant sysadmins (who check before trying to connect
>>> something) will be surprised.
>>
>> Ok I can add this back, I didn't see anybody using it, but again I didn't 
>> look
>> very hard.
>
> Thank you.
>
>>> - What happens if userspace tries to connect an already-connected device
>>>   to some other server? Currently that can't happen (you get EBUSY);
>>>   with this patch, I believe it can, and data corruption would be the
>>>   result (on *two* nbd devices). Additionally, with the loss of the pid
>>>   attribute (as above) and the ensuing loss of the -check functionality,
>>>   this might actually be a somewhat likely scenario.
>>
>> Once you do DO_IT then you'll get the EBUSY, so no problems.
>
> Oh, okay. I missed that part.
>
>> Now if you modify the client to connect to two different servers then yes you
>> could have data corruption, but hey if you do stupid things then bad things
>> happen, I'm not sure we need to explicitly keep this from happening.
>
> Yeah, totally agree there.
>
>>> - What happens if one of the multiple connections drop but the others do
>>>   not?
>>
>> It keeps on trucking, but the connections that break will return -EIO.  
>> That's
>> not good, I'll fix it to tear down everything if that happens.
>
> Right. Alternatively, you could perhaps make it so that the lost
> connection is removed, unack'd requests on that connection are resent,
> and the session moves on with one less connection (unless the lost
> connection is the last one, in which case we die as before). That might
> be too much work and not worth it though.

Yeah I wasn't sure if we could just randomly remove hw queue's in blk mq while 
the device is still up.  If that is in fact easy to do then I'm in favor of 
trucking along with less connections than we originally had, otherwise I think 
it'll be too big of a pain.  Also some users (Facebook in this case) would 
rather the whole thing fail so we can figure out what went wrong rather than 
suddenly going at a degraded performance.

>
>>> - This all has the downside that userspace now has to predict how many
>>>   parallel connections will be necessary and/or useful. If the initial
>>>   guess was wrong, we don't have a way to correct later on.
>>
>> No, it relies on the admin to specify based on their environment.
>
> Sure, but I suppose it would be nice if things could dynamically grow
> when needed, and/or that the admin could modify the number of
> connections of an already-connected device. Then again, this might also
> be too much work and not worth it.

I mean we can set some magic number, like num_connections = min(nr_cpus, 4); 
and 
then that way people who aren't paying attention suddenly are going faster.  I 
think anything smarter and we'd have to figure out how fast the link is and at 
what point we're hitting the diminishing returns and that path lies sadness.

>
> [...]
>>> A better way, long term, would presumably be to modify the protocol to
>>> allow multiplexing several requests in one NBD session. This would deal
>>> with what you're trying to fix too[1], while it would not pull in all of
>>> the above problems.
>>>
>>> [1] after all, we have to serialize all traffic anyway, just before it
>>> heads into the NIC.
>>
>> Yeah I considered changing the protocol to handle multiplexing different
>> requests, but that runs into trouble since we can't guarantee that each 
>> discrete
>> sendmsg/recvmsg is going to atomically copy our buffer in.  We can accomplish
>> this with KCM of course which is a road I went down for a little while, but 
>> then
>> we have the issue of the actual data to send across, and KCM is limited to a
>> certain buffer size (I don't remember what it was exactly).  This limitation 
>> is
>> fine in practice I think, but I got such good performance with multiple
>> connections that I threw all that work away and went with this.
>
> Okay, sounds like you've given that way more thought than me, and that
> that's a dead end. Never mind then.
>

Not necessarily a dead end, but a path that requires a lot of thought and 
experimentation to figure out if it's worth it.  So maybe I'll waste an interns 
summer on figuring it out, but I've got other things I'd rather do ;).  Thanks,

Josef

Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

On Fri, Sep 09, 2016 at 04:36:07PM -0400, Josef Bacik wrote:
> On 09/09/2016 04:02 PM, Wouter Verhelst wrote:
[...]
> > I see some practical problems with this:
> > - You removed the pid attribute from sysfs (unless you added it back and
> >   I didn't notice, in which case just ignore this part). This kills
> >   userspace in two ways:
> >   - systemd/udev mark an NBD device as "not active" if the sysfs pid
> > attribute is absent. Removing that attribute causes the new nbd
> > systemd unit to stop working.
> >   - nbd-client -check relies on this attribute too, which means that
> > even if people don't use systemd, their init scripts will still
> > break, and vigilant sysadmins (who check before trying to connect
> > something) will be surprised.
> 
> Ok I can add this back, I didn't see anybody using it, but again I didn't 
> look 
> very hard.

Thank you.

> > - What happens if userspace tries to connect an already-connected device
> >   to some other server? Currently that can't happen (you get EBUSY);
> >   with this patch, I believe it can, and data corruption would be the
> >   result (on *two* nbd devices). Additionally, with the loss of the pid
> >   attribute (as above) and the ensuing loss of the -check functionality,
> >   this might actually be a somewhat likely scenario.
> 
> Once you do DO_IT then you'll get the EBUSY, so no problems.

Oh, okay. I missed that part.

> Now if you modify the client to connect to two different servers then yes you
> could have data corruption, but hey if you do stupid things then bad things
> happen, I'm not sure we need to explicitly keep this from happening.

Yeah, totally agree there.

> > - What happens if one of the multiple connections drop but the others do
> >   not?
> 
> It keeps on trucking, but the connections that break will return -EIO.  
> That's 
> not good, I'll fix it to tear down everything if that happens.

Right. Alternatively, you could perhaps make it so that the lost
connection is removed, unack'd requests on that connection are resent,
and the session moves on with one less connection (unless the lost
connection is the last one, in which case we die as before). That might
be too much work and not worth it though.

> > - This all has the downside that userspace now has to predict how many
> >   parallel connections will be necessary and/or useful. If the initial
> >   guess was wrong, we don't have a way to correct later on.
> 
> No, it relies on the admin to specify based on their environment.

Sure, but I suppose it would be nice if things could dynamically grow
when needed, and/or that the admin could modify the number of
connections of an already-connected device. Then again, this might also
be too much work and not worth it.

[...]
> > A better way, long term, would presumably be to modify the protocol to
> > allow multiplexing several requests in one NBD session. This would deal
> > with what you're trying to fix too[1], while it would not pull in all of
> > the above problems.
> >
> > [1] after all, we have to serialize all traffic anyway, just before it
> > heads into the NIC.
> 
> Yeah I considered changing the protocol to handle multiplexing different 
> requests, but that runs into trouble since we can't guarantee that each 
> discrete 
> sendmsg/recvmsg is going to atomically copy our buffer in.  We can accomplish 
> this with KCM of course which is a road I went down for a little while, but 
> then 
> we have the issue of the actual data to send across, and KCM is limited to a 
> certain buffer size (I don't remember what it was exactly).  This limitation 
> is 
> fine in practice I think, but I got such good performance with multiple 
> connections that I threw all that work away and went with this.

Okay, sounds like you've given that way more thought than me, and that
that's a dead end. Never mind then.

> Thanks for the review, I'll fix up these issues you've pointed out and resend,

Thanks,

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
   people in the world who think they really understand all of its rules,
   and pretty much all of them are just lying to themselves too.
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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Josef Bacik]

On 09/09/2016 04:02 PM, Wouter Verhelst wrote:
> Hi Josef,
>
> On Thu, Sep 08, 2016 at 05:12:05PM -0400, Josef Bacik wrote:
>> Apologies if you are getting this a second time, it appears vger ate my last
>> submission.
>>
>> --
>>
>> This is a patch series aimed at bringing NBD into 2016.  The two big 
>> components
>> of this series is converting nbd over to using blkmq and then allowing us to
>> provide more than one connection for a nbd device.  The NBD user space server
>> doesn't care about how many connections it has to a particular device, so we 
>> can
>> easily open multiple connections to the server and allow blkmq to handle
>> multi-plexing over the different connections.
>
> I see some practical problems with this:
> - You removed the pid attribute from sysfs (unless you added it back and
>   I didn't notice, in which case just ignore this part). This kills
>   userspace in two ways:
>   - systemd/udev mark an NBD device as "not active" if the sysfs pid
> attribute is absent. Removing that attribute causes the new nbd
> systemd unit to stop working.
>   - nbd-client -check relies on this attribute too, which means that
> even if people don't use systemd, their init scripts will still
> break, and vigilant sysadmins (who check before trying to connect
> something) will be surprised.

Ok I can add this back, I didn't see anybody using it, but again I didn't look 
very hard.

> - What happens if userspace tries to connect an already-connected device
>   to some other server? Currently that can't happen (you get EBUSY);
>   with this patch, I believe it can, and data corruption would be the
>   result (on *two* nbd devices). Additionally, with the loss of the pid
>   attribute (as above) and the ensuing loss of the -check functionality,
>   this might actually be a somewhat likely scenario.

Once you do DO_IT then you'll get the EBUSY, so no problems.  Now if you modify 
the client to connect to two different servers then yes you could have data 
corruption, but hey if you do stupid things then bad things happen, I'm not 
sure 
we need to explicitly keep this from happening.

> - What happens if one of the multiple connections drop but the others do
>   not?

It keeps on trucking, but the connections that break will return -EIO.  That's 
not good, I'll fix it to tear down everything if that happens.

> - This all has the downside that userspace now has to predict how many
>   parallel connections will be necessary and/or useful. If the initial
>   guess was wrong, we don't have a way to correct later on.

No, it relies on the admin to specify based on their environment.

>
> My suggestion is to reject an additional connection unless it comes from
> the same userspace process as the previous connections, and to retain
> the pid attribute (since it is now guaranteed to be the same for all the
> connections). That should fix the first two issues (while unfortunately
> reinforcing the last one). The third would also need to have clearly
> defined semantics, at the very least.

Yeah that sounds reasonable to me, I hadn't thought of some other pid trying to 
setup a device at the same time.

>
> A better way, long term, would presumably be to modify the protocol to
> allow multiplexing several requests in one NBD session. This would deal
> with what you're trying to fix too[1], while it would not pull in all of
> the above problems.
>
> [1] after all, we have to serialize all traffic anyway, just before it
> heads into the NIC.
>

Yeah I considered changing the protocol to handle multiplexing different 
requests, but that runs into trouble since we can't guarantee that each 
discrete 
sendmsg/recvmsg is going to atomically copy our buffer in.  We can accomplish 
this with KCM of course which is a road I went down for a little while, but 
then 
we have the issue of the actual data to send across, and KCM is limited to a 
certain buffer size (I don't remember what it was exactly).  This limitation is 
fine in practice I think, but I got such good performance with multiple 
connections that I threw all that work away and went with this.

Thanks for the review, I'll fix up these issues you've pointed out and resend,

Josef

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Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

[Wouter Verhelst]

Hi Josef,

On Thu, Sep 08, 2016 at 05:12:05PM -0400, Josef Bacik wrote:
> Apologies if you are getting this a second time, it appears vger ate my last
> submission.
> 
> --
> 
> This is a patch series aimed at bringing NBD into 2016.  The two big 
> components
> of this series is converting nbd over to using blkmq and then allowing us to
> provide more than one connection for a nbd device.  The NBD user space server
> doesn't care about how many connections it has to a particular device, so we 
> can
> easily open multiple connections to the server and allow blkmq to handle
> multi-plexing over the different connections.

I see some practical problems with this:
- You removed the pid attribute from sysfs (unless you added it back and
  I didn't notice, in which case just ignore this part). This kills
  userspace in two ways:
  - systemd/udev mark an NBD device as "not active" if the sysfs pid
attribute is absent. Removing that attribute causes the new nbd
systemd unit to stop working.
  - nbd-client -check relies on this attribute too, which means that
even if people don't use systemd, their init scripts will still
break, and vigilant sysadmins (who check before trying to connect
something) will be surprised.
- What happens if userspace tries to connect an already-connected device
  to some other server? Currently that can't happen (you get EBUSY);
  with this patch, I believe it can, and data corruption would be the
  result (on *two* nbd devices). Additionally, with the loss of the pid
  attribute (as above) and the ensuing loss of the -check functionality,
  this might actually be a somewhat likely scenario.
- What happens if one of the multiple connections drop but the others do
  not?
- This all has the downside that userspace now has to predict how many
  parallel connections will be necessary and/or useful. If the initial
  guess was wrong, we don't have a way to correct later on.

My suggestion is to reject an additional connection unless it comes from
the same userspace process as the previous connections, and to retain
the pid attribute (since it is now guaranteed to be the same for all the
connections). That should fix the first two issues (while unfortunately
reinforcing the last one). The third would also need to have clearly
defined semantics, at the very least.

A better way, long term, would presumably be to modify the protocol to
allow multiplexing several requests in one NBD session. This would deal
with what you're trying to fix too[1], while it would not pull in all of
the above problems.

[1] after all, we have to serialize all traffic anyway, just before it
heads into the NIC.

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
   people in the world who think they really understand all of its rules,
   and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12

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