From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark Overmeer Subject: Re: raid 0 read performance Date: Thu, 29 Dec 2005 12:04:13 +0100 Message-ID: <20051229110413.GD17602@earth.overmeer.net> References: <5d96567b0512290210q2f4b63c5tc91a49523c93e4df@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <5d96567b0512290210q2f4b63c5tc91a49523c93e4df@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: "Raz Ben-Jehuda(caro)" Cc: Linux RAID Mailing List , Neil Brown List-Id: linux-raid.ids * Raz Ben-Jehuda(caro) (raziebe@gmail.com) [051229 10:10]: > I have tested the overhead of linux raid0. > I used two scsi atlas maxtor disks ( 147 MB) and combined them to single > raid0 volume. > The raid is striped in 256K stripes. Are you sure you tested "linux" overhead? Maybe you have just tested raid0 properties. > I filled the raid0 up to the maximum with files over xfs file system. > I've checked the peformance of reading 60 files like that: > > while need to read > for every file > read 0.5 M from file > > I got 50 MB/s . > > Armed with this knowledge i went and did the same test over > one disk and i got 32 MB/s . > > Question: > why is this perfomance drop ? Good performance for such small reads wrt the block-size... Ok, simple calculation: seek time average for one disk is half a rotation for two disks 1 - (1-0.5)*(1-0.5) = 0.75 rotation Without taking into account the time to do actual reads: 0.5/0.75 * (2x32MB/s) = 43MB/s (34% performance drop) The only effect you see is that the probability that both disks are in optimal position to read from them decreases. Solution: take very large files wrt the stripe-size to get double performance. Or take files smaller than the stripe-size. Of course, there can be other reasons which can reduce the performance as well. However, I achieve 200MB/s over 4 striped disk each capable of 50MB/s for huge files and 64K stipes... Linux doesn't seem to be the bottleneck in my setup. -- Regards, MarkOv ------------------------------------------------------------------------ Mark Overmeer MSc MARKOV Solutions Mark@Overmeer.net solutions@overmeer.net http://Mark.Overmeer.net http://solutions.overmeer.net