Re: dm-kcopyd: Delayed unplug of the queues

[Mike Snitzer <snitzer@redhat.com>]

On Sun, Oct 24 2010 at  9:03pm -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> dm-kcopyd: Delayed unplug of the queues
> 
> This patch improves performance aby about 20% when writing to the snapshot
> origin. We keep track of two block devices to unplug (one for read and the
> other for write) and unplug them when exiting "do_work" thread. If there are
> more devices used (in theory it could happen, in practice it is rare), we
> unplug immediatelly.

Again, more specifics on how you realized this 20% performance would be
appreciated.  I'd imagine this patch is really where the increased
latency comes back (due to removing immediate unplug)?

But does the fact that >2 devices will unplug immediately mitigate the
worst of the higher latency (e.g. for DM mirrors with >2 legs)?

Mike