From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthieu Patou Subject: Random IO with md raid Date: Sat, 05 Dec 2009 01:31:22 +0300 Message-ID: <4B198DBA.5000906@matws.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids Hello, I'm reviewing one new server with a 3ware 9690 controller where I've got a couple of sata drives connected. In order to see how software raid behave in comparaison with a the raid controller I setup 3 volume: * 1 raid 1 volume of 2 1TB hard drive * 2 single volume of 1 TB hard drive each I assemble the 2 single volume in a raid 1 volume Both are formatted in XFS mounted noatime and nobarrier. Sequentials reads and writes are quite similar between software raid and 3ware raid. But when it comes to random writes and random reads I find something strange. The memory is 1G so I made iozone use 2G file with 4k block On 3ware (in operation by second) Random read: 193 Random write: 1232 On md Random read: 178 Random write: 361 First I am bit worried that I can manage to get such good random result with the 3ware card I guess I am seeing some cache effect (there is 256 MB on the card) Also I'm surprised that the software raid is not able to take advantage of this cache effect in the same proportion (1 to 4). I've been searching internet for tips on raid tunning but it turns out it's mostly for sequential access (ie. like readahead). Does anyone has an any idea ? Matthieu.