On 4/30/18 1:25 PM, Luis R. Rodriguez wrote: > On Mon, Apr 30, 2018 at 12:07:31PM -0600, Jens Axboe wrote: >> On 4/30/18 11:19 AM, Brian Foster wrote: >>> On Mon, Apr 30, 2018 at 09:32:52AM -0600, Jens Axboe wrote: >>>> XFS recently added support for async discards. While this can be >>>> a win for some workloads and devices, there are also cases where >>>> async bursty discard will severly harm the latencies of reads >>>> and writes. >>>> >>>> Add a 'discard_sync' mount flag to revert to using sync discard, >>>> issuing them one at the time and waiting for each one. This fixes >>>> a big performance regression we had moving to kernels that include >>>> the XFS async discard support. >>>> >>>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> >>>> --- >>> >>> Hm, I figured the async discard stuff would have been a pretty clear win >>> all around, but then again I'm not terribly familiar with what happens >>> with discards beneath the fs. I do know that the previous behavior would >>> cause fs level latencies due to holding up log I/O completion while >>> discards completed one at a time. My understanding is that this lead to >>> online discard being pretty much universally "not recommended" in favor >>> of fstrim. >> >> It's not a secret that most devices suck at discard. > > How can we know if a device sucks at discard? I was going to ask the same thing. ;) "Meh, punt to the admin!" I'm having deja vu but can't remember why. Seems like this has come up before and we thought it should be a block device tunable, not pushed down from the filesystem. Is that possible? -Eric
