From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joe Thornber Subject: Re: [PATCH 1/2] dm-kcopyd: introduce per-module throttle structure Date: Fri, 3 Jun 2011 12:01:12 +0100 Message-ID: <20110603110111.GA4969@ubuntu> References: <20110601095106.GA3718@ubuntu> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: device-mapper development Cc: "Alasdair G. Kergon" List-Id: dm-devel.ids 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