From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tomasz Chmielewski Subject: Re: Propose of enhancement of raid1 driver Date: Sat, 21 Oct 2006 18:01:05 +0200 Message-ID: <453A4441.1040409@wpkg.org> References: <45347D6E.3000600@mieszczak.com.pl> <17718.61672.794099.425723@cse.unsw.edu.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <17718.61672.794099.425723@cse.unsw.edu.au> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: Miroslaw Mieszczak , linux-raid@vger.kernel.org List-Id: linux-raid.ids Neil Brown wrote: > On Tuesday October 17, mirek@mieszczak.com.pl wrote: >> I would like to propose an enhancement of raid 1 driver in linux kernel. >> The enhancement would be speedup of data reading on mirrored partitions. >> The idea is easy. >> If we have mirrored partition over 2 disks, and these disk are in sync, there is >> possibility of simultaneous reading of the data from both disks on the same way >> as in raid 0. So it would be chunk1 read from master, chunk2 read from slave at >> the same time. >> As result it would give significant speedup of read operation (comparable with >> speed of raid 0 disks). > > This is not as easy as it sounds. > Skipping over blocks within a track is no faster than reading blocks > in the track, so you would need to make sure that your chunk size is > larger than one track - probably it would need to be several tracks. What you said is certainly true when we read one file at a given moment. What if we read two different files at a given time? Certainly, it would be faster if DRIVE_1 reads FILE_1, and DRIVE_2 reads FILE_2. > Raid1 already does some read-balancing, though it is possible (even > likely) that it doesn't balance very effectively. Working out how > best to do the balancing in general in a non-trivial task, but would > be worth spending time on. Probably what I said before isn't very correct, as RAID-1 has no idea of the filesystem that is on top of it; rather, it will see attempts to access differend areas of the array? -- Tomasz Chmielewski http://wpkg.org