From mboxrd@z Thu Jan 1 00:00:00 1970 From: david@lang.hm Subject: Re: limits on raid Date: Mon, 18 Jun 2007 11:12:45 -0700 (PDT) Message-ID: References: <18034.479.256870.600360@notabene.brown> <18034.3676.477575.490448@notabene.brown> <20070616020320.GB2002@animx.eu.org> <18035.23867.576212.859440@notabene.brown> <4676BEC2.7090809@redhat.com> <20070618180327.GP10008@csclub.uwaterloo.ca> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Return-path: In-Reply-To: <20070618180327.GP10008@csclub.uwaterloo.ca> Sender: linux-raid-owner@vger.kernel.org To: Lennart Sorensen Cc: Brendan Conoboy , Neil Brown , Wakko Warner , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org List-Id: linux-raid.ids On Mon, 18 Jun 2007, Lennart Sorensen wrote: > On Mon, Jun 18, 2007 at 10:28:38AM -0700, david@lang.hm wrote: >> I plan to test the different configurations. >> >> however, if I was saturating the bus with the reconstruct how can I fire >> off a dd if=/dev/zero of=/mnt/test and get ~45M/sec whild only slowing the >> reconstruct to ~4M/sec? >> >> I'm putting 10x as much data through the bus at that point, it would seem >> to proove that it's not the bus that's saturated. > > dd 45MB/s from the raid sounds reasonable. > > If you have 45 drives, doing a resync of raid5 or radi6 should probably > involve reading all the disks, and writing new parity data to one drive. > So if you are writing 5MB/s, then you are reading 44*5MB/s from the > other drives, which is 220MB/s. If your resync drops to 4MB/s when > doing dd, then you have 44*4MB/s which is 176MB/s or 44MB/s less read > capacity, which surprisingly seems to match the dd speed you are > getting. Seems like you are indeed very much saturating a bus > somewhere. The numbers certainly agree with that theory. > > What kind of setup is the drives connected to? simple ultra-wide SCSI to a single controller. I didn't realize that the rate reported by /proc/mdstat was the write speed that was takeing place, I thought it was the total data rate (reads + writes). the next time this message gets changed it would be a good thing to clarify this. David Lang