[dm-devel] [PATCH 1/2] dm-kcopyd: introduce per-module throttle structure
Joe Thornber
thornber at redhat.com
Fri Jun 3 11:01:12 UTC 2011
On Thu, Jun 02, 2011 at 03:55:16PM -0400, Mikulas Patocka wrote:
> > iv) you haven't explained how the sys admin works out the correct
> > throttle value.
>
> There is no "correct" value. The "correct" value depends on how important
> is copying itself v.s. other i/o.
So who is going to set this? Do you really have no advice for them
beyond 'there is no correct value'?
> In theory (if disk scheduler were perfect), we wouldn't need any
> throttling. The disk scheduler should recognize that the kcopyd process is
> sending way more requests than any other process and should lower the
> i/o priority of kcopyd process.
>
> In practice, the disk scheduler doesn't do it well, so kcopyd hurts the
> users. If you want an automated fix, fix the disk scheduler. But don't put
> disk scheduler logic into device mapper --- it dosn't belong there.
I totally agree with these two paragraphs. Any throttling you add to
kcopyd is always going to be a hack.
- Joe
More information about the dm-devel
mailing list