On Fri, 2009-08-14 at 18:19 -0600, Chris Worley wrote:

On Fri, Aug 14, 2009 at 5:45 PM, Matthew Wilcox[off-list ref] wrote:

quoted

On Fri, Aug 14, 2009 at 05:21:32PM -0600, Chris Worley wrote:

quoted

Sooner is better than waiting to coalesce.  The longer an LBA is
inactive, the better for any management scheme.  If you wait until
it's reused, you might as well forgo the advantages of TRIM/UNMAP.  If
a the controller wants to coalesce, let it coalesce.

I'm sorry, you're wrong.  There is a tradeoff point, and it's different
for each drive model.  Sending down a steady stream of tiny TRIMs is
going to give terrible performance.

Sounds like you might be using junk for a device?

For junk, a little coalescing may be warranted... like in the I/O
schedular, but no more than 100usecs wait before posting, or then you
effect high performing devices too.

Um, I think you missed the original point in all of this at the
beginning of the thread:  On ATA TRIM commands cannot be tagged.  This
means you have to drain the outstanding NCQ commands (stalling the
device) before you can send a TRIM.   If we do this for every discard,
the performance impact will be pretty devastating, hence the need to
coalesce.  It's nothing really to do with device characteristics, it's
an ATA protocol problem.

James