On Wed, Jun 20, 2012 at 06:06:31AM +1000, Dave Chinner wrote: > > 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. Actually, ext4 is also sending the discards during (well, actually, after) the commit which frees the extent/inode. We do aggregate them while the commit is open, but once the transaction is committed, we send out the discards. I suspect the difference is in the granularity of the transactions between ext4 and xfs. > 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... What Dave said. :-) This is true for both ext4 and xfs. As a result, I can very easily see there being a distinction made between when we *do* want to pass the discards all the way down to the device, and when we only want the thinp layer to process them --- because for current devices, sending discards down to the physical device is very heavyweight. I'm not sure how we could do this without a nasty layering violation, but some way in which we could label fstrim discards versus "we've committed the unlink/truncate and so thinp can feel free to reuse these blocks" discards would be interesting to consider. - Ted -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel