[Gluster-devel] Eager-lock and nfs graph generation

[Anand Avati]

On Tue, Feb 19, 2013 at 5:12 PM, Anand Avati <anand.avati at gmail.com> wrote:

>
>
> On Tue, Feb 19, 2013 at 3:59 AM, Pranith Kumar K <pkarampu at 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.avati at gmail.com>wrote:
>>
>>>
>>>
>>>  On Mon, Feb 11, 2013 at 9:32 PM, Pranith Kumar K <pkarampu at 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
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