I would still recommend sharding. Imagine that you got 2 TB disks for a VM and
one of the oVirt hosts needs maintenance.When gluster has to heal that 2TB
file, your VM won't be able to access the file for a very long time and will
fail. Sharding is important for having no-downtime maintenance.
Yet I have one question.Did you use preallocated VM disks or you used thin
provisioning for the qcow2 ?
Best Regards,Strahil Nikolov
Description of problem:
Intermittent VM pause and Qcow image corruption after add new bricks.
I'm suffered an issue on image corruption on oVirt 4.3 caused by default
gluster ovirt profile, and intermittent VM pause. the problem is similar to
#2246 #2254 in glusterfs issue and VM pause issue report in ovirt user group.
The gluster vol did not have pending heal object, vol appear in good shape, xfs
is healthy, no hardware issue. Sadly few VM have mystery corruption after new
bricks added.
Afterwards, I try to simulate the problem with or without
"cluster.lookup-optimize off" few time, but the problem is not 100%
reproducible with lookup-optimize on, I got 1 of 3 attempt that able to
reproduce it. It really depend on the workloads and cache status at that moment
and the number of object after rebalance as well.
Also I tried to disable all sharding features, it ran very solid, write
performance increase by far, no corruption, no VM pause when the gluster under
stress.
So, here is a decision question on shard or not shard.
IMO, even recommendation document saying it break large file into smaller chunk
that allow healing to complete faster, a larger file can spread over multiple
bricks. But there are uncovered issue compared to full large file in this case,
I'd like to further deep dive into the reason why recommend shard as default
for oVirt? Especially from the reliability and performance perspective,
sharding seems losing this end for ovirt/kvm workloads. Is it more appropriate
to just tell ovirt user to ensure underlying single bricks shall be large
enough to hold the largest chunk instead? Besides, anything i'm overlooked for
the shard setting? I'm really doubt to enable sharding on the volume after
disaster.
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