From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: doesm mdadm try to use fastest HDD ? Date: Fri, 02 Nov 2007 09:10:23 -0400 Message-ID: <472B21BF.4060201@tmr.com> References: <20071102125205.52c2d115@absurd> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20071102125205.52c2d115@absurd> Sender: linux-raid-owner@vger.kernel.org To: Janek Kozicki Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Janek Kozicki wrote: > Hello, > > My three HHDs have following speeds: > > hda - speed 70 MB/sec > hdc - speed 27 MB/sec > sda - speed 60 MB/sec > > They create a raid1 /dev/md0 and raid5 /dev/md1 arrays. I wanted to > ask if mdadm is trying to pick the fastest HDD during operation? > > Maybe I can "tell" which HDD is preferred? > If you are doing raid-1 between hdc and some faster drive, you could try using write-mostly and see go that works for you. For raid-5, it's faster to read the data off the slow drive than reconstruct it with multiple reads to multiple othjer faster drives. > This came to my mind when I saw this: > > # mdadm --query --detail /dev/md1 | grep Prefer > > Preferred Minor : 1 > > And also in the manual: > > -W, --write-mostly [...] "can be useful if mirroring over a slow link." > > > many thanks for all your help! > I have two thoughts on this: 1 - if performance is critical, replace the slow drive 2 - for most things you do, I would expect seek to be more important than transfer rate -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979