[Gluster-devel] [Gluster-users] POSIX locks and disconnections between clients and bricks

Wed Mar 27 13:08:08 UTC 2019

On Wed, Mar 27, 2019 at 1:13 PM Pranith Kumar Karampuri <pkarampu at redhat.com>
wrote:

>
>
> On Wed, Mar 27, 2019 at 5:13 PM Xavi Hernandez <jahernan at redhat.com>
> wrote:
>
>> On Wed, Mar 27, 2019 at 11:52 AM Raghavendra Gowdappa <
>> rgowdapp at redhat.com> wrote:
>>
>>>
>>>
>>> On Wed, Mar 27, 2019 at 12:56 PM Xavi Hernandez <jahernan at redhat.com>
>>> wrote:
>>>
>>>> Hi Raghavendra,
>>>>
>>>> On Wed, Mar 27, 2019 at 2:49 AM Raghavendra Gowdappa <
>>>> rgowdapp at redhat.com> wrote:
>>>>
>>>>> All,
>>>>>
>>>>> Glusterfs cleans up POSIX locks held on an fd when the client/mount
>>>>> through which those locks are held disconnects from bricks/server. This
>>>>> helps Glusterfs to not run into a stale lock problem later (For eg., if
>>>>> application unlocks while the connection was still down). However, this
>>>>> means the lock is no longer exclusive as other applications/clients can
>>>>> acquire the same lock. To communicate that locks are no longer valid, we
>>>>> are planning to mark the fd (which has POSIX locks) bad on a disconnect so
>>>>> that any future operations on that fd will fail, forcing the application to
>>>>> re-open the fd and re-acquire locks it needs [1].
>>>>>
>>>>
>>>> Wouldn't it be better to retake the locks when the brick is reconnected
>>>> if the lock is still in use ?
>>>>
>>>
>>> There is also  a possibility that clients may never reconnect. That's
>>> the primary reason why bricks assume the worst (client will not reconnect)
>>> and cleanup the locks.
>>>
>>
>> True, so it's fine to cleanup the locks. I'm not saying that locks
>> shouldn't be released on disconnect. The assumption is that if the client
>> has really died, it will also disconnect from other bricks, who will
>> release the locks. So, eventually, another client will have enough quorum
>> to attempt a lock that will succeed. In other words, if a client gets
>> disconnected from too many bricks simultaneously (loses Quorum), then that
>> client can be considered as bad and can return errors to the application.
>> This should also cause to release the locks on the remaining connected
>> bricks.
>>
>> On the other hand, if the disconnection is very short and the client has
>> not died, it will keep enough locked files (it has quorum) to avoid other
>> clients to successfully acquire a lock. In this case, if the brick is
>> reconnected, all existing locks should be reacquired to recover the
>> original state before the disconnection.
>>
>>
>>>
>>>> BTW, the referenced bug is not public. Should we open another bug to
>>>> track this ?
>>>>
>>>
>>> I've just opened up the comment to give enough context. I'll open a bug
>>> upstream too.
>>>
>>>
>>>>
>>>>
>>>>>
>>>>> Note that with AFR/replicate in picture we can prevent errors to
>>>>> application as long as Quorum number of children "never ever" lost
>>>>> connection with bricks after locks have been acquired. I am using the term
>>>>> "never ever" as locks are not healed back after re-connection and hence
>>>>> first disconnect would've marked the fd bad and the fd remains so even
>>>>> after re-connection happens. So, its not just Quorum number of children
>>>>> "currently online", but Quorum number of children "never having
>>>>> disconnected with bricks after locks are acquired".
>>>>>
>>>>
>>>> I think this requisite is not feasible. In a distributed file system,
>>>> sooner or later all bricks will be disconnected. It could be because of
>>>> failures or because an upgrade is done, but it will happen.
>>>>
>>>> The difference here is how long are fd's kept open. If applications
>>>> open and close files frequently enough (i.e. the fd is not kept open more
>>>> time than it takes to have more than Quorum bricks disconnected) then
>>>> there's no problem. The problem can only appear on applications that open
>>>> files for a long time and also use posix locks. In this case, the only good
>>>> solution I see is to retake the locks on brick reconnection.
>>>>
>>>
>>> Agree. But lock-healing should be done only by HA layers like AFR/EC as
>>> only they know whether there are enough online bricks to have prevented any
>>> conflicting lock. Protocol/client itself doesn't have enough information to
>>> do that. If its a plain distribute, I don't see a way to heal locks without
>>> loosing the property of exclusivity of locks.
>>>
>>
>> Lock-healing of locks acquired while a brick was disconnected need to be
>> handled by AFR/EC. However, locks already present at the moment of
>> disconnection could be recovered by client xlator itself as long as the
>> file has not been closed (which client xlator already knows).
>>
>
> What if another client (say mount-2) took locks at the time of disconnect
> from mount-1 and modified the file and unlocked? client xlator doing the
> heal may not be a good idea.
>

To avoid that we should ensure that any lock/unlocks are sent to the
client, even if we know it's disconnected, so that client xlator can track
them. The alternative is to duplicate and maintain code both on AFR and EC
(and not sure if even in DHT depending on how we want to handle some cases).

A similar thing could be done for open fd, since the current solution
duplicates code in AFR and EC, but this is another topic...

>
>>
>> Xavi
>>
>>
>>> What I proposed is a short term solution. mid to long term solution
>>> should be lock healing feature implemented in AFR/EC. In fact I had this
>>> conversation with +Karampuri, Pranith <pkarampu at redhat.com> before
>>> posting this msg to ML.
>>>
>>>
>>>>
>>>>> However, this use case is not affected if the application don't
>>>>> acquire any POSIX locks. So, I am interested in knowing
>>>>> * whether your use cases use POSIX locks?
>>>>> * Is it feasible for your application to re-open fds and re-acquire
>>>>> locks on seeing EBADFD errors?
>>>>>
>>>>
>>>> I think that many applications are not prepared to handle that.
>>>>
>>>
>>> I too suspected that and in fact not too happy with the solution. But
>>> went ahead with this mail as I heard implementing lock-heal  in AFR will
>>> take time and hence there are no alternative short term solutions.
>>>
>>
>>>
>>>> Xavi
>>>>
>>>>
>>>>>
>>>>> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1689375#c7
>>>>>
>>>>> regards,
>>>>> Raghavendra
>>>>>
>>>>> _______________________________________________
>>>>> Gluster-users mailing list
>>>>> Gluster-users at gluster.org
>>>>> https://lists.gluster.org/mailman/listinfo/gluster-users
>>>>
>>>>
>
> --
> Pranith
>
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