On Tue, Jun 19, 2012 at 02:48:59PM -0400, Mike Snitzer wrote: > On Tue, Jun 19 2012 at 10:44am -0400, > Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > > > On Tue, Jun 19 2012 at 9:52am -0400, > > Spelic <spelic@xxxxxxxxxxxxx> wrote: > > > > > I do not know what is the mechanism for which xfs cannot unmap > > > blocks from dm-thin, but it really can't. > > > If anyone has dm-thin installed he can try. This is 100% > > > reproducible for me. > > > > I was initially surprised by this considering the thinp-test-suite does > > test a compilebench workload against xfs and ext4 using online discard > > (-o discard). > > > > But I just modified that test to use a thin-pool with 'ignore_discard' > > and the test still passed on both ext4 and xfs. > > > > So there is more work needed in the thinp-test-suite to use blktrace > > hooks to verify that discards are occuring when the compilebench > > generated files are removed. > > > > I'll work through that and report back. > > blktrace shows discards for both xfs and ext4. > > But in general xfs is issuing discards with much smaller extents than > ext4 does, e.g.: THat's normal when you use -o discard - XFS sends extremely fine-grained discards as the have to be issued during the checkpoint commit that frees the extent. Hence they can't be aggregated like is done in ext4. As it is, no-one really should be using -o discard - it is extremely inefficient compared to a background fstrim run given that discards are unqueued, blocking IOs. It's just a bad idea until the lower layers get fixed to allow asynchronous, vectored discards and SATA supports queued discards... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
