[dm-devel] Ext4 and xfs problems in dm-thin on allocation and discard

[Christoph Hellwig]

On Wed, Jun 20, 2012 at 06:39:38AM +1000, Dave Chinner wrote:
> Exactly - XFS transactions are fine grained, checkpoints are coarse.
> We don't merge extents freed in fine grained transactions inside
> checkpoints. We probably could, but, well, it's complex to do in XFS
> and merging adjacent requests is something the block layer is
> supposed to do....

Last time I checked it actually tries to do that for discard requests,
but then badly falls flat (=oopses).  That's the reason why the XFS
transaction commit code still uses the highly suboptimal synchronous
blkdev_issue_discard instead of the async variant I wrote when designing
the code.

Another "issue" with the XFS discard pattern and the current block
layer implementation is that XFS frees a lot of small metadata like
inode clusters and btree blocks and discards them as well.  If those
simply fill one of the vectors in a range ATA TRIM command and/or a
queueable command that's not much of an issue, but with the current
combination of non-queueable, non-vetored TRIM that's a fairly nasty
pattern.

So until the block layer is sorted out I can not recommend actually
using -o dicard.  I planned to sort out the block layer issues ASAP
when writing that code, but other things have kept me busy every since.