Re: [PATCH] dm-kcopyd: monitor io activity

[Joe Thornber <ejt@redhat.com>]

On Mon, May 30, 2011 at 12:40:39PM -0400, Mikulas Patocka wrote:
> Here I'm sending three patches that limit kcopyd speed. There is global 
> limit for all kcopyds running in a system. The user can set percentage of 
> kcopyd speed in /sys/module/dm_mod/parameters/dm_kcopyd_throttle

Could you give some explanation of how to choose an appropriate value
for this please?  Should it be 100% unless there is an issue?  How
does the sys admin spot such issues?

Different kcopyd clients have different performance requirements, for
instance the initial sync of a new mirror leg may have less priority
than a copy-on-write exception for a snapshot.  This isn't something
we've addressed so far, but if you're starting to slow kcopyd down
(i.e. so the mirror sync doesn't saturate the system), then I think
you'll get some complaints from the snapshot users.

- joe