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

Jason Dillaman jdillama at redhat.com
Sat Nov 18 05:08:37 PST 2017

Can you capture a blktrace while perform fstrim to record the discard
operations? A 1TB trim extent would cause a huge impact since it would
translate to approximately 262K IO requests to the OSDs (assuming 4MB
backing files).

On Fri, Nov 17, 2017 at 6:19 PM, Brendan Moloney <moloney at ohsu.edu> wrote:
> 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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