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

[Pranith Kumar K]

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.
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.

Pranith.
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