[Gluster-devel] AFR: Fail lookups when quorum not met

[Niels de Vos]

On Fri, Sep 22, 2017 at 12:27:46PM +0530, Ravishankar N wrote:
> Hello,
> 
> In AFR we currently allow look-ups to pass through without taking into
> account whether the lookup is served from the good or bad brick. We always
> serve from the good brick whenever possible, but if there is none, we just
> serve the lookup from one of the bricks that we got a positive reply from.
> 
> We found a bug  [1] due to this behavior were the iatt values returned in
> the lookup call was bad and caused the client to hang. The proposed fix [2]
> was to fail look ups when we definitely know the lookup can't be trusted (by
> virtue of AFR xattrs indicating the replies we got from the up bricks are
> indeed bad).
> 
> Note that this fix is *only* for replica 3 or arbiter volumes (not replica
> 2, where there is no notion of quorum). But we want to 'harden' the fix by 
> not allowing any look ups at all if quorum is not met (or) it is met but
> there are no good copies.
> 
> Some implications of this:
> 
> -If a file ends up in data/meta data split-brain in replica 3/arbiter (rare
> occurrence), we won't be able to delete it from the mount.
> 
> -Even if the only brick that is up is the good copy, we still fail it due to
> lack of quorum.
> 
> Does any one have comments/ feedback?

I think additional improvements for correctness outweigh the two
negative side-effects that you listed.

Possibly the 2nd point could get some confusion from users. "it always
worked before" may be a reason to add a volume option for this? That is
something you can consider, but if you deem that overkill then I'm ok
with that too.

Thanks,
Niels


> 
> Thanks,
> 
> Ravi
> 
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1467250
> 
> [2] https://review.gluster.org/#/c/17673/ (See review comments on the
> landing page if interested)
> 

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