On Tue, Feb 19, 2013 at 6:11 PM, Pranith Kumar K <pkara...@redhat.com>wrote:
> On 02/20/2013 07:03 AM, Anand Avati wrote: > > > > On Tue, Feb 19, 2013 at 5:12 PM, Anand Avati <anand.av...@gmail.com>wrote: > >> >> >> On Tue, Feb 19, 2013 at 3:59 AM, Pranith Kumar K <pkara...@redhat.com>wrote: >> >>> On 02/19/2013 11:26 AM, Anand Avati wrote: >>> >>> Thinking over this, looks like there is a problem! >>> >>> Write-behind guarantees: That a second write request arriving after the >>> acknowledgement of a first overlapping request (whether written-behind or >>> otherwise) will be guaranteed to be fulfilled in the backend in the same >>> order (i.e, the second overlapping request will be "serialized" behind the >>> first one in the fulfillment process) >>> >>> Eager-lock requirement: That write-behind will send no two write >>> requests on an overlapping region at the same time. >>> >>> The requirement-set and guarantee-set have a big overlap, but the >>> requirement-set is not a subset. >>> >>> This is because of O_SYNC writes. write-behind performs >>> write-serialization at fulfillment only for written behind requests (which >>> get covered under the conflict detection code during liability >>> fulfillment). However, if two threads (or apps) issue overlapping O_SYNC >>> writes to the same region at approx same time, then write-behind will let >>> both of them go by without any kind of serialization, into eager lock, >>> violating the assumptions! >>> >>> I'm wondering if it is a safer idea to implement overlap checks within >>> eager-lock code itself rather than depend on write-behind :| >>> >>> Avati >>> >>> On Mon, Feb 11, 2013 at 10:07 PM, Anand Avati <anand.av...@gmail.com>wrote: >>> >>>> >>>> >>>> On Mon, Feb 11, 2013 at 9:32 PM, Pranith Kumar K >>>> <pkara...@redhat.com>wrote: >>>> >>>>> hi, >>>>> Please note that this is a case in theory and I did not run into such >>>>> situation, but I feel it is important to address this. >>>>> Configuration with 'Eager-lock on" and "write-behind off" should not >>>>> be allowed as it leads to lock synchronization problems which lead to data >>>>> in-consistency among replicas in nfs. >>>>> lets say bricks b1, b2 are in replication. >>>>> Gluster Nfs server uses 1 anonymous fd to perform all write-fops. If >>>>> eager-lock is enabled in afr, the lock-owner is used as fd's address which >>>>> will be same for all write-fops, so there will never be any inodelk >>>>> contention. If write-behind is disabled, there can be writes that overlap. >>>>> (Does nfs makes sure that the ranges don't overlap?) >>>>> >>>>> Now imagine the following scenario: >>>>> lets say w1, w2 are 2 write fops on same offset and length. w1 with >>>>> all '0's and w2 with all '1's. If these 2 write fops are executed in 2 >>>>> different threads, the order of arrival of write fops on b1 can be w1, w2 >>>>> where as on b2 it is w2, w1 leading to data inconsistency between the two >>>>> replicas. The lock contention will not happen as both lk-owner, transport >>>>> are same for these 2 fops. >>>>> >>>> >>>> Write-behind has to functions - a) performing operations in the >>>> background and b) serializing overlapping operations. >>>> >>>> While the problem does exist, the specifics are different from what >>>> you describe. since all writes coming in from NFS will always use the same >>>> anonymous FD, two near-in-time/overlapping writes will never contend with >>>> inodelk() but instead the second write will inherit the lock and changelog >>>> from the first. In either case, it is a problem. >>>> >>>> >>>>> We can add a check in glusterd for volume set to disallow such >>>>> configuration, BUT by default write-behind is off in nfs graph and by >>>>> default eager-lock is on. So we should either turn on write-behind for nfs >>>>> or turn off eager-lock by default. >>>>> >>>>> Could you please suggest how to proceed with this if you agree that I >>>>> did not miss any important detail that makes this theory invalid. >>>>> >>>> >>>> It seems loading write-behind xlator in NFS graph looks like a >>>> simpler solution. eager-locking is crucial for replicated NFS write >>>> performance. >>>> >>>> Avati >>>> >>> >>> Shall we disable eager-lock for files opened with O_SYNC, for now? >>> >> >> Bad news: the problem is slightly worse than just this. Even with >> non-O_SYNC writes, there is a possibility in write-behind where, if a >> second overlapping write request comes so close to the first request that, >> if wb_enqueue() of the second one happens after wb_enqueue() of the first >> write, but before any unwind() after the first wb_enqueue() (i.e >> wb_inode->gen is not bumped), then the two write requests can be wound down >> together to eager lock. >> >> > But this has a simple fix - http://review.gluster.org/4550. Disabling > eager-locking for O_SYNC files is a bad idea. We absolutely want > eager-locking for O_SYNC files. Thinking more.. > > Avati > > Why is disabling eager-lock for O_SYNC files a bad idea? It is acceptable > to sacrifice a bit of performance for O_SYNC isn't it? > s/bit/quite a bit/. For O_SYNC writes, eager locking is the only saving grace in performance as write-behind stays out of the way completely. We would need overlap checks either in AFR or write-behind for O_SYNC writes. Avati
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