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

[Anand Avati]

On Tue, Feb 19, 2013 at 6:11 PM, Pranith Kumar K <pkarampu at 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.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
>
> 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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