From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Raid6 write performance Date: Fri, 13 Mar 2009 13:11:47 -0400 Message-ID: <49BA93D3.9070700@tmr.com> References: <49742E74.9090502@rabbit.us> <49A88DB4.40008@zytor.com> <49AADFCA.2060406@rabbit.us> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <49AADFCA.2060406@rabbit.us> Sender: linux-raid-owner@vger.kernel.org To: Peter Rabbitson Cc: "H. Peter Anvin" , linux-raid@vger.kernel.org List-Id: linux-raid.ids Peter Rabbitson wrote: > 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. > That's a lovely theory, but in practice I have to say I have never measured any such thing, using benchmarks intended to match real world, or even heavy disk writes of a dumb nature like dd. I have tested through the raw device, and through filesystems, tuned stripe-cache-size and buffers, tried setting "stride" in ext3, all to conclude that with raid5 I see essentially write speed of 1x a single drive and read speed of (N-1)x as you suggest. Actually, looking at results for arrays with more drives I can see a trend to write at (N/3)x speed, being a seek-write for full chunk data and seef-read-write for XOR. But even on six drive arrays I don't get near (N-1)x in measurable. -- Bill Davidsen "Woe unto the statesman who makes war without a reason that will still be valid when the war is over..." Otto von Bismark