On Sat, 2009-08-15 at 09:22 -0400, Mark Lord wrote:

James Bottomley wrote:

quoted

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.

..

I don't think that's really much of an issue -- we already have to do
that for cache-flushes whenever barriers are enabled.  Yes it costs,
but not too much.

That's not really what the enterprise is saying about flush barriers.
True, not all the performance problems are NCQ queue drain, but for a
steady workload they are significant.

James