Date: Wed, 5 Sep 2001 16:23:23 +0100 From: "Lawrence Farr" <l.farr@epcdirect.co.uk> To: "'Doug Hardie'" <bc979@lafn.org> Cc: "'Greg Lehey'" <grog@FreeBSD.ORG>, "'Lawrence Farr'" <lawrence@epcdirect.co.uk>, "'David Gilbert'" <dgilbert@velocet.ca>, "'Chris BeHanna'" <behanna@zbzoom.net>, "'FreeBSD-Stable'" <stable@FreeBSD.ORG> Subject: RE: [stable] Re: RAID5 Message-ID: <003401c1361e$b52bf440$c80aa8c0@lfarr> In-Reply-To: <f04330116b7bbe195e610@[10.0.1.100]>
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Errr. No. One's read ones write. >-----Original Message----- >From: Doug Hardie [mailto:bc979@lafn.org] >Sent: 05 September 2001 15:16 >To: Lawrence Farr >Cc: 'Greg Lehey'; 'Lawrence Farr'; 'David Gilbert'; 'Chris >BeHanna'; 'FreeBSD-Stable' >Subject: RE: [stable] Re: RAID5 > > >At 9:26 +0100 9/5/01, Lawrence Farr wrote: >>I did these tests in response to a recent thread on hardware (look for >>"3ware stuff not ready for heavy duty useage"). >> >> >On Tuesday, 4 September 2001 at 13:52:02 +0100, Lawrence >Farr wrote: >>>> Just to add another benchmark, I got: >>>> >>>> Pass 23 - 1048576 kb written in 115 seconds, at 9118 kb/Sec >>>> Pass 23 - 1048576 kb read in 15 seconds, at 69905 kb/Sec >>> >>>This shows there's a big difference. Which is which? What is it >> >really doing here? > >Perhaps some experiences I had with several years running large RAID >5 systems may be of help. I used a RAID 5 configuration where each >bay had 7 disks for striping and parity data. The 8th disk was a >fast-write/recovery disk. If all the drives were working correctly >then writes to the system were first written to the fast-write drive >and the I/O terminated. The RAID system then distributed the >information to the 7 data disks. If the fast-write disk was not >full, then this ran a little faster than read speed. > >However, we frequently did large Oracle tablespace initializations. >Generally all 7 disks were used for the tablespace. We were able to >monitor disk usage on all the drives plus the line from the computer >to the RAID system. During the first few minutes, all the drives >(fast-write and data) would be running a 100%. The line from the >computer to the RAID system was operating at very close to 100%. >However, after just short of 2 GB transfered (the size of the >fast-write disk), the line to the RAID dropped to zero transfers. It >would stay at zero for about 10 minutes. Then a few blocks would be >transferred and back to sleep. It continued like this till the >tablespace was initialized. The average line utilization was very >low. The disk utilization was at 100%. > >Writes to the fast-write disk are fast. Distributing the information >the RAID 5 way to multiple disks is slower and hence the RAID system >bottlenecked and quit accepting data from the host. It had no place >to put it until the fast-write disk had been processed. While the >information above is not complete enough to say for sure, it looks >like thats what occurred in that test. >-- >-- Doug > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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