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

Jason Dillaman jdillama at redhat.com
Mon Nov 27 17:55:40 PST 2017

Correct -- the optimization is in Luminous and not in Jewel.

On Mon, Nov 27, 2017 at 8:53 PM, Brendan Moloney <moloney at ohsu.edu> wrote:
> Thanks Jason.
> I am running Jewel currently.  I do have object-map enabled, but it sounds like the no-op discard optimization is specific to the newer librbd?
> Thanks again,
> Brendan
> ________________________________________
> From: Jason Dillaman [jdillama at redhat.com]
> Sent: Monday, November 27, 2017 5:49 PM
> To: Brendan Moloney
> Cc: ceph-users at lists.ceph.com
> Subject: Re: [ceph-users] I/O stalls when doing fstrim on large RBD
> My only possible suggestion would be to try a Luminous librbd client
> and enable the object-map feature on the image. When the object map is
> enabled, librbd will optimize away all the no-op discard requests. If
> that doesn't work, it could be an issue in the OS / controller
> interaction.
> On Mon, Nov 27, 2017 at 5:56 PM, Brendan Moloney <moloney at ohsu.edu> wrote:
>> Hi,
>> Anyone have input on this?  I am surprised there are not more people running into this issue.  I guess most people don't have multi-TB RBD images?  I think ext4 might also fair better since it does keep track of blocks that have been discarded in the past and not modified so that they don't get discarded again.
>> For the benefit of anyone else on the list who is following along, I went ahead and made the blktrace output available: https://filebin.ca/3imcZ5IHImxW/fstrim_blktrace.tar.gz
>> Here is the script I wrote to chunk up the fstrim: https://gist.github.com/moloney/5763a02e3847a5368af56110cc583544
>> Using this script and the "cfq" scheduler I do seem to have things running a bit more smoothly. I also raised the disk timeout values from 30 seconds to 180.  I am still not convinced that the issue is resolved though, I will need to wait and see what happens when a VM under heavy load does an fstrim.
>> Thanks,
>> Brendan
> --
> Jason


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