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

[Joe Thornber <thornber@redhat.com>]

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