Hi,
I guess this isn't strictly about Ceph, but I feel like other folks here must
have run into the same issues.
I am trying to keep my thinly provisioned RBD volumes thin. I use virtio-scsi
to attach the RBD volumes to my VMs with the "discard=unmap" option. The RBD is
formatted as XFS and some of them can be quite large (16TB+). I have a cron
job that runs "fstrim" commands twice a week in the evenings.
The issue is that I see massive I/O stalls on the VM during the fstrim. To the
point where I am getting kernel panics from hung tasks and other timeouts. I
have tried a number of things to lessen the impact:
- Switching from deadline to CFQ (initially I thought this helped, but now
I am not convinced)
- Running fstrim with "ionice -c idle" (this doesn't seem to make a
difference)
- Chunking the fstrim with the offset/length options (helps reduce worst
case, but I can't trim less than 1TB at a time and that can still cause a pause
for several minutes)
Is there anything else I can do to avoid this issue?
Thanks,
Brendan
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