Re: mdadm raid1 read performance
From: Keld Jørn Simonsen <hidden>
Date: 2011-05-05 11:10:06
On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote:
On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos [off-list ref] wrote:quoted
as a separate question, what should be the theoretical performance of raid5?x(N-1) So a 4 drive RAID5 should read at 3 time the speed of a single drive.
Actually, theoretically, it should be more than that for reading, more like N minus some overhead. In a raid5 stripe of 4 disks, when reading you do not read the checksum block, and thus you should be able to have all 4 drives occupied with reading real data. Some benchmarks back this up, http://home.comcast.net/~jpiszcz/20080329-raid/ http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-26255_10.html The latter reports a 3.44 times performance for raid5 reads with 4 disks, significantly over the N-1 = 3.0 mark. For writing, you are correct with the N-1 formular. best regards keld