From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: dm-kcopyd: Delayed unplug of the queues
Date: Fri, 19 Nov 2010 17:18:31 -0500
Message-ID: <20101119221830.GB25730@redhat.com>
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To: Mikulas Patocka <mpatocka@redhat.com>
Cc: dm-devel@redhat.com, Alasdair G Kergon <agk@redhat.com>
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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