On Sun, Feb 17, 2019 at 06:42:59PM -0500, Ric Wheeler wrote: > On 2/17/19 4:09 PM, Dave Chinner wrote: > >On Sun, Feb 17, 2019 at 03:36:10PM -0500, Ric Wheeler wrote: > >>One proposal for btrfs was that we should look at getting discard > >>out of the synchronous path in order to minimize the slowdown > >>associated with enabling discard at mount time. Seems like an > >>obvious win for "hint" like operations like discard. > >We already have support for that. blkdev_issue_discard() is > >synchornous, yes, but __blkdev_issue_discard() will only build the > >discard bio chain - it is up to the caller to submit and wait for it. > > > >Some callers (XFS, dm-thinp, nvmet, etc) use a bio completion to > >handle the discard IO completion, hence allowing async dispatch and > >processing of the discard chain without blocking the caller. Others > >(like ext4) simply call submit_bio_wait() to do wait synchronously > >on completion of the discard bio chain. > > > >>I do wonder where we stand now with the cost of the various discard > >>commands - how painful is it for modern SSD's? > >AIUI, it still depends on the SSD implementation, unfortunately. > > I think the variability makes life really miserable for layers above it. Yup, that it does. > Might be worth constructing some tooling that we can use to validate > or shame vendors over That doesn't seem to work. > - testing things like a full device discard, > discard of fs block size and big chunks, discard against already > discarded, etc. We did that many years ago because discard on SSDs sucked: https://people.redhat.com/lczerner/discard/test_discard.html https://sourceforge.net/projects/test-discard/files/ And, really, that didn't changed a thing - discard still sucks... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
