From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Rabbitson Subject: Re: Raid6 write performance Date: Sun, 01 Mar 2009 20:19:38 +0100 Message-ID: <49AADFCA.2060406@rabbit.us> References: <49742E74.9090502@rabbit.us> <49A88DB4.40008@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <49A88DB4.40008@zytor.com> Sender: linux-raid-owner@vger.kernel.org To: "H. Peter Anvin" Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids H. Peter Anvin wrote: > Peter Rabbitson wrote: >> Hi, >> >> I am experimenting with raid6 on 4 drives on 2.6.27.11. The problem I am >> having is that no matter what chunk size I use, the write benchmark >> always comes out at single drive speed, although I should be seeing >> double drive speed (read speed is at near 4x as expected). > > I have no idea why you "should" be seeing double drive speed. All > drives have to be written, so you'd logically see single drive speed. > Because with properly adjusted elevators and chunk sizes it is reasonable to expect N * S write speed from _any_ raid, where N is the number of different data bearing disks in a stripe, and S is the speed of a hard drive (assuming the drive speeds are equal). So for raid5 we have N = numdisks-1, for raid6 numdisks-2, for raid10 -n4 -pf3 we get 4-(3-1) and so on. I have personally verified the write behavior for raid10 and raid5, don't see why it should/would be different for raid6. In any case the witnessed problem was due to a hardware misconfiguration, which has not been resolved to this day. Thus thread is dead :)