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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@xxxxxxxxx>
wrote:
On Mon, Feb 11, 2013 at 9:32 PM, Pranith
Kumar K <pkarampu@xxxxxxxxxx>
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?
Pranith
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