[ceph-users] I/O stalls when doing fstrim on large RBD

Brendan Moloney moloney at ohsu.edu
Fri Nov 17 15:19:34 PST 2017


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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