On 05/05/2012 08:02 AM, Anand Avati wrote:
On Wed, May 2, 2012 at 3:55 AM, Xavier Hernandez
<[email protected] <mailto:[email protected]>> wrote:
Hello,
I'm wondering if there are any requisites that translators must
satisfy to work correctly inside glusterfs.
In particular I need to know two things:
1. Are translators required to respect the order in which they
receive the requests ?
This is specially important in translators such as
performance/io-threads or caching ones. It seems that these
translators can reorder requests. If this is the case, is there
any way to force some order between requests ? can inodelk/entrylk
be used to force the order ?
Translators are not expected to maintain ordering of requests. The
only translator which takes care of ordering calls is write-behind.
After acknowledging back write requests it has to make sure future
requests see the true "effect" as though the previous write actually
completed. To that end, it queues future "dependent" requests till the
write acknowledgement is received from the server.
inodelk/entrylk calls help achieve synchronization among clients (by
getting into a critical section) - just like a mutex. It is an
arbitrator. It does not help for ordering of two calls. If one call
must strictly complete after another call from your translator's point
of view (i.e, if it has such a requirement), then the latter call's
STACK_WIND must happen in the callback of the former's STACK_UNWIND
path. There are no guarantees maintained by the system to ensure that
a second STACK_WIND issued right after a first STACK_WIND will
complete and callback in the same order. Write-behind does all its
ordering gimmicks only because it STACK_UNWINDs a write call
prematurely and therefore must maintain the causal effects by means of
queueing new requests behind the downcall towards the server.
Good to know
2. Are translators required to propagate callback arguments even
if the result of the operation is an error ? and if an internal
translator error occurs ?
Usually no. If op_ret is -1, only op_errno is expected to be a usable
value. Rest of the callback parameters are junk.
When a translator has multiple subvolumes, I've seen that some
arguments, such as xdata, are replaced with NULL. This can be
understood, but are regular translators (those that only have one
subvolume) allowed to do that or must they preserve the value of
xdata, even in the case of an internal error ?
It is best to preserve the arguments unless you know specifically what
you are doing. In case of error, all the non-op_{ret,errno} arguments
are typically junk, including xdata.
If this is not a requisite, xdata loses it's function of
delivering back extra information.
Can you explain? Are you seeing a use case for having a valid xdata in
the callback even with op_ret == -1?
As a part of a translator that I'm developing that works with multiple
subvolumes, I need to implement some healing support to mantain data
coherency (similar to AFR). After some thought, I decided that it could
be advantageous to use a dedicated healing translator located near the
bottom of the translators stack on the servers. This translator won't
work by itself, it only adds support to be used by a higher level
translator, which have to manage the logic of the healing and decide
when a node needs to be healed.
To do this, sometimes I need to return an error because an operation
cannot be completed due to some condition related with healing itself
(not with the underlying storage). However I need to send some specific
healing information to let the upper translator know how it has to
handle the detected condition.
I cannot send a success answer because intermediate translators could
take the fake data as valid and they could begin to operate incorrectly
or even create inconsistencies. The other alternative is to use op_errno
to encode the extra data, but this will also be difficult, even
impossible in some cases, due to the amount of data and the complexity
to combine it with an error code without mislead intermediate
translators with strange or invalid error codes.
I talked with John Mark about this translator and he suggested me to
discuss it over the list. Therefore I'll initiate another thread to
expose in more detail how it works and I would appreciate very much your
opinion, and that of the other developers, about it. Especially if it
can really be faster/safer that other solutions or not, or if you find
any problem or have any suggestion to improve it. I think it could also
be used by AFR and any future translator that may need some healing
capabilities.
Thank you very much,
Xavi
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