Date: Tue, 01 Apr 2003 09:26:04 -0500 From: Jason Andresen <jandrese@mitre.org> To: "Michael C. Brenner" <mbrenner@kaibren.com> Cc: FreeBSD Stable List <freebsd-stable@freebsd.org> Subject: Re: vinum performance Message-ID: <3E89A17C.6030003@mitre.org> In-Reply-To: <5.2.0.9.2.20030331214429.02677a90@gw.kaibren.com> References: <3E88AECD.10607@liwing.de> <20030330125138.K23911@leelou.in.tern> <3E870CC7.5000204@mac.com> <20030330175605.E23911@leelou.in.tern> <3E87204C.5060304@ludd.luth.se> <3E88524A.1060600@mitre.org> <3E88AECD.10607@liwing.de> <5.2.0.9.2.20030331214429.02677a90@gw.kaibren.com>
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Michael C. Brenner wrote: > Writing to a RAID5 stripe set requires that all disks in the array > successfully report completion before the RAID5 controller's buffer can > be released back to the cache. (Applies to either software or hardware > raid.) If you are doing a large block write (like dd) you can easily > fill the cache on most controllers. Once the cache is full, the > controller slows each write to the LONGEST completion time of each > spindle in the array. ECC calculation becomes part of the latency also. > In a 5 drive system (other than one where the cache is larger than the > largest file being written as in a large EMC array) the writes are > always about 4-5 times longer than the reads. Tuning stripes and > blocking factors can speed up a specific transfer but RAID5 has always > been slow to write large data and best for read mostly data. Ahh, that makes sesnse. > Read operations benefit from RAID5 or mirrors. Now the shortest > completion time of the minimal drive set is the gating event. The first > set of drives to deliver the data block ends the operation. This makes a > 2 to 1 difference into a 4 to one difference. Thanks for clearing this up. -- \ |_ _|__ __|_ \ __| Jason Andresen jandrese@mitre.org |\/ | | | / _| Network and Distributed Systems Engineer _| _|___| _| _|_\___| Office: 703-883-7755
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