On Wed, Mar 27, 2019 at 12:56 PM Xavi Hernandez <jaher...@redhat.com> wrote:
> Hi Raghavendra, > > On Wed, Mar 27, 2019 at 2:49 AM Raghavendra Gowdappa <rgowd...@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 To communicate that locks are no longer valid, we 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. > 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. 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 <pkara...@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@gluster.org >> https://lists.gluster.org/mailman/listinfo/gluster-users > >
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