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

Thu Mar 28 02:04:48 UTC 2019

On Wed, Mar 27, 2019 at 8:38 PM Xavi Hernandez <jahernan at redhat.com> wrote:

> On Wed, Mar 27, 2019 at 2:20 PM Pranith Kumar Karampuri <
> pkarampu at redhat.com> wrote:
>
>>
>>
>> On Wed, Mar 27, 2019 at 6:38 PM Xavi Hernandez <jahernan at redhat.com>
>> wrote:
>>
>>> 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 To communicate that locks are no longer valid, we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).
>>>
>>
>> Didn't understand the solution. I wanted to highlight that client xlator
>> by itself can't make a decision about healing locks because it doesn't know
>> what happened on other replicas. If we have replica-3 volume and all 3
>> bricks get disconnected to their respective bricks. Now another mount
>> process can take a lock on that file modify it and unlock. Now upon
>> reconnection, the old mount process which had locks would think it always
>> had the lock if client xlator independently tries to heal its own locks
>> because file is not closed on it so far. But that is wrong. Let me know if
>> it makes sense....
>>
>
> My point of view is that any configuration with these requirements will
> have an appropriate quorum value so that it's impossible to have two or
> more partitions of the nodes working at the same time. So, under this
> assumptions, mount-1 can be in two situations:
>
> 1. It has lost a single brick and it's still operational. The other bricks
> will continue locked and everything should work fine from the point of view
> of the application. Any other application trying to get a lock will fail
> due to lack of quorum. When the lost brick comes back and is reconnected,
> client xlator will still have the fd reference and locks taken (unless the
> application has released the lock or closed the fd, in which case client
> xlator should get notified and clear that information), so it should be
> able to recover the previous state.
>
> 2. It has lost 2 or 3 bricks. In this case mount-1 has lost quorum and any
> operation going to that file should fail with EIO. AFR should send a
> special request to client xlator so that it forgets any fd's and locks for
> that file. If bricks reconnect after that, no fd reopen or lock recovery
> will happen. Eventually the application should close the fd and retry
> later. This may succeed to not, depending on whether mount-2 has taken the
> lock already or not.
>
> So, it's true that client xlator doesn't know the state of the other
> bricks, but it doesn't need to as long as AFR/EC strictly enforces quorum
> and updates client xlator when quorum is lost.
>

Just curious. Is there any reason why you think delegating the actual
responsibility of re-opening or forgetting the locks to protocol/client is
better when compared to AFR/EC doing the actual work of re-opening files
and reacquiring locks? Asking this because, in the case of plain
distribute, DHT will also have to indicate Quorum loss on every disconnect
(as Quorum consisted of just 1 brick).

>From what I understand, the design is the same one which me, Pranith, Anoop
and Vijay had discussed (in essence) but  varies in implementation details.

> I haven't worked out all the details of this approach, but I think it
> should work and it's simpler to maintain than trying to do the same for AFR
> and EC.
>
> Xavi
>
>
>>
>>> 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
>>>>
>>>
>>
>> --
>> Pranith
>>
>
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