Re: Ext4 and xfs problems in dm-thin on allocation and discard

[Ted Ts'o <tytso@mit.edu>]

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