From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Raid6 write performance Date: Mon, 26 Jan 2009 13:47:05 -0500 Message-ID: <497E0529.3050501@tmr.com> References: <49742E74.9090502@rabbit.us> <8CB482475EFC01A-17F0-256E@webmail-dx12.sysops.aol.com> <1cd4f41f0901190107l53d02785l844da71a6643605@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1cd4f41f0901190107l53d02785l844da71a6643605@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: NiftyFedora Mitch Cc: thomas62186218@aol.com, rabbit+list@rabbit.us, linux-raid@vger.kernel.org List-Id: linux-raid.ids NiftyFedora Mitch wrote: > On Mon, Jan 19, 2009 at 12:10 AM, wrote: > >> I tested RAID 5 and RAID 6 with 12 x 15K SAS drives on Ubuntu 8.04 64-bit >> and found their performance to be about the same. I used 256 K chunk size, >> v1.0 superblocks, stripecachesize of 16384, and readahead of 65536. >> >> RAID 5 reads: 774 MB/sec >> RAID 5 writes: 585 MB/sec >> >> RAID 6 reads: 742 MB/sec >> RAID 6 writes: 559 MB/sec >> >> My CPU utilization remains under 10% though during writes, and I'm wondering >> what can be done to get write performance closer to read performance. I have >> dual quad-core CPUs so there's plenty of CPU to go around. Any ideas on that >> front? >> >> -Thomas >> >> >> -----Original Message----- >> From: Peter Rabbitson >> To: linux-raid@vger.kernel.org >> Sent: Sun, 18 Jan 2009 11:40 pm >> Subject: Raid6 write performance >> > > >> 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). Is there some >> hidden setting that I am overlooking, or is this a current known >> limitation of raid6? In contrast if I make a raid5 on these 4 drives, I >> get the expected 3xdrive write speed, and occasionally 4xdrive linear >> read speed. >> >> When the write test is running, I get about 14% of system cpu a sporadic >> 40% of iowait and the rest idle at all times (machine is in runlevel 1 >> so not to screw with results). Anyone has any ideas? >> > > Read bandwidth will always be quicker than writes with parity. > Data and parity both need to be written in an atomic way so the completion > will be gated by the last write to be "posted as done" back to the > system. > > It can pay to locate the journal on a physically different, smaller > and faster resource. > One informative experiment might be to mount the file system as ext2 and compare > and contrast with the same FS mounted as ext3. I am not recommending ext2 > over ext3 other than as an experiment to see what the impact of the > journal activity is... > > Also see noatime, relatime, etc. -- 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