On Fri, Jun 03 2011 at 7:01am -0400, Joe Thornber <thornber@xxxxxxxxxx> wrote: > 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. Wouldn't it be better to tie in to the block layer's new throttling infrastructure that Vivek added? Possibly expose a callback that enables kcopyd consuming devices to throttle kcopyd as a side-effect of the higher-level throttle? Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
