Re: [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup

[Max Reitz]

On 2017-11-09 15:16, Vladimir Sementsov-Ogievskiy wrote:
> This is needed to implement image-fleecing scheme, when we create
> a temporary node, mark our active node to be backing for the temp,
> and start backup(sync=none) from active node to the temp node.
> Temp node then represents a kind of snapshot and may be used
> for external backup through NBD.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy 
> ---
> 
> What was the reason to abandon non-root nodes?
> 
>  blockdev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 56a6b24a0b..d0a5a107f0 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -3345,7 +3345,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, 
> BlockJobTxn *txn,
>  backup->compress = false;
>  }
>  
> -bs = qmp_get_root_bs(backup->device, errp);
> +bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>  if (!bs) {
>  return NULL;
>  }

I can't find anything wrong with it, but it will need some more changes.

One is the comment in bdrv_backing_attach() that unsets the
BLOCK_OP_TYPE_BACKUP_SOURCE blocker which claims that it's safe to do so
because a backing file can never be a backup source anyway.  Question
here: Why did that seem to be important?

Other places to look into are all callers of bdrv_op_block_all() that do
not unblock BLOCK_OP_TYPE_BACKUP_SOURCE afterwards (i.e. all but
bdrv_backing_attach()).  I suspect this is because they were not sure
whether they could guarantee consistent data...  But does that extend to
their children?  (Because the exact node that is blocked will still be
seen as blocked by the backup job, so that is safe.)  This wouldn't give
us any data corruption, though, because the user just gets garbage data
in their backup -- their fault for using a garbage node, I'd argue.

Maybe it doesn't matter at all.  If you really want to you can probably
already attach some raw node to any node you want to backup and hey,
that's a root node, so congratulations, you can run blockdev-backup on
it.  Well, I mean, you can run it but it won't do you any good because
the before_write notifiers don't work then.

(Same for mirror, by the way -- the dirty bitmap of that raw node
wouldn't get set.)

((By the way, I don't suppose that's how it should work...  But I don't
suppose that we want propagation of dirtying towards the BDS roots, do
we? :-/))


Finally, there is the BlockdevBackup and DriveBackup QAPI documentation
that would each need one "root" removed.

Max



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Re: [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup

[Kevin Wolf]

Am 09.11.2017 um 17:33 hat Eric Blake geschrieben:
> On 11/09/2017 08:16 AM, Vladimir Sementsov-Ogievskiy wrote:
> > This is needed to implement image-fleecing scheme, when we create
> > a temporary node, mark our active node to be backing for the temp,
> > and start backup(sync=none) from active node to the temp node.
> > Temp node then represents a kind of snapshot and may be used
> > for external backup through NBD.
> > 
> > Signed-off-by: Vladimir Sementsov-Ogievskiy 
> > ---
> > 
> > What was the reason to abandon non-root nodes?
> 
> I think the original restriction was that we didn't know all the
> implications to having multiple readers to an intermediate node, so it
> was easier to prevent it with plans to add it later than to add it up
> front and deal with the fallout.  But I think that now we are
> sufficiently versed in handling BDS trees with multiple readers, with
> proper op-blocking in place; so you are right that we can probably
> support it now.

Op blockers are actually the reason why I'm not so sure. The old
blockers often still only work on the top level, and we haven't fully
replaced them yet. In particular, graph modifications might not be
protected well enough by the new system yet.

But then, backup works only on a single node in the source tree, not on
a whole subchain, so I don't see what other operation could conflict
with a backup job. The only thing is resizes, but that is covered by the
new system.

So in the end, I think this specific change should be okay.

> However, I'm a bit worried that there is no documentation change about
> this in a .json QAPI file, nor any easy way to introspect via QMP
> whether a particular qemu implementation supports this (short of trying
> it and seeing whether it works).  I'm also thinking that this is 2.12
> material, unless we can prove it is fixing a bug for 2.11 that was not
> previously present.

Sounds like another use case for the capability annotations that Markus
wants to introduce for commands in the QAPI schema.

Kevin


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Re: [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup

[Eric Blake]

On 11/09/2017 08:16 AM, Vladimir Sementsov-Ogievskiy wrote:
> This is needed to implement image-fleecing scheme, when we create
> a temporary node, mark our active node to be backing for the temp,
> and start backup(sync=none) from active node to the temp node.
> Temp node then represents a kind of snapshot and may be used
> for external backup through NBD.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy 
> ---
> 
> What was the reason to abandon non-root nodes?

I think the original restriction was that we didn't know all the
implications to having multiple readers to an intermediate node, so it
was easier to prevent it with plans to add it later than to add it up
front and deal with the fallout.  But I think that now we are
sufficiently versed in handling BDS trees with multiple readers, with
proper op-blocking in place; so you are right that we can probably
support it now.

However, I'm a bit worried that there is no documentation change about
this in a .json QAPI file, nor any easy way to introspect via QMP
whether a particular qemu implementation supports this (short of trying
it and seeing whether it works).  I'm also thinking that this is 2.12
material, unless we can prove it is fixing a bug for 2.11 that was not
previously present.

> 
>  blockdev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 56a6b24a0b..d0a5a107f0 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -3345,7 +3345,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, 
> BlockJobTxn *txn,
>  backup->compress = false;
>  }
>  
> -bs = qmp_get_root_bs(backup->device, errp);
> +bs = bdrv_lookup_bs(backup->device, backup->device, errp);
>  if (!bs) {
>  return NULL;
>  }
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3266
Virtualization:  qemu.org | libvirt.org



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[Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup

[Vladimir Sementsov-Ogievskiy]

This is needed to implement image-fleecing scheme, when we create
a temporary node, mark our active node to be backing for the temp,
and start backup(sync=none) from active node to the temp node.
Temp node then represents a kind of snapshot and may be used
for external backup through NBD.

Signed-off-by: Vladimir Sementsov-Ogievskiy 
---

What was the reason to abandon non-root nodes?

 blockdev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index 56a6b24a0b..d0a5a107f0 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3345,7 +3345,7 @@ BlockJob *do_blockdev_backup(BlockdevBackup *backup, 
BlockJobTxn *txn,
 backup->compress = false;
 }
 
-bs = qmp_get_root_bs(backup->device, errp);
+bs = bdrv_lookup_bs(backup->device, backup->device, errp);
 if (!bs) {
 return NULL;
 }
-- 
2.11.1