Re: [PATCH 1/2] dm-kcopyd: introduce per-module throttle structure

[Joe Thornber <thornber@redhat.com>]

On Sat, Jun 11, 2011 at 04:27:02PM -0400, Mikulas Patocka wrote:
> It is much easier to explain to the users "if you set X value in 
> /sys/module/dm_mirror/parameters/raid1_resync_throttle, then the copying 
> will be done in X% of time, leaving the disk idle 100-X% of time", then to 
> invent some magic mechanisms that change multiple things based on X and 
> other conditions.

Yes, that is very easy to explain to the users.  This sort of
description is what I was after when I kept asking you to tell me how
to set it.

It's not clear to me that setting the throttle to 80% will copy at 80%
of the speed, or leave the disk idle for 20% of the time.  Perhaps you
have some benchmarks to back this up?  (with different memory pressure
situations please).

Restricting the cpu available to issue io is not a great way to
throttle io.  There will be plenty of situations where 20% of the cpu
is enough to swamp the devices.

- Joe