Re: [patch 2/2] dm-delay: add third flush class

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Apr 16 2018 at  6:33pm -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> This patch adds a new class for dm-delay that delays flush requests.
> Previously, flushes were delayed as writes, but it caused problems if the
> user needed to create a device with one or few slow sectors for the
> purpose of testing - all flushes would be forwarded to this device and
> delayed, and that skews the test results. This patch allows to select 0
> delay for flushes.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

The flush device shouldn't ever be allowed to be different than the
write device should it?  Also, what does an offset even mean in the
context of flush?

Pretty awkward really.  I get that you've factored out the ctr code and
are just reusing it for flush; and that in practice these knobs won't
get used (or flush_device won't be different than write_device).. but
I'm just not following why we want to expose flush_offset and
flush_device at all.

Mike