From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: Typical RAID5 transfer speeds Date: Sat, 19 Dec 2009 11:43:20 +0000 Message-ID: <4B2CBC58.5020902@anonymous.org.uk> References: <200912190205.04407.bs_lists@aakef.fastmail.fm> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <200912190205.04407.bs_lists@aakef.fastmail.fm> Sender: linux-raid-owner@vger.kernel.org To: Bernd Schubert Cc: Linux RAID List-Id: linux-raid.ids On 19/12/2009 01:05, Bernd Schubert wrote: > On Saturday 19 December 2009, Matt Tehonica wrote: >> I have a 4 disk RAID5 using a 2048K chunk size and using XFS > > 4 disks is a bad idea. You should have 2^n data disks, but you have 2^1 + 1 = > 3 data disks. As parity information are calculated in the power of two and > blocks are written in the power of two Sorry, but where did you get that from? p = d1 xor d2 xor d3 has nothing to do with powers of two, and I'm sure blocks are written whenever they need to be, not in powers of two. > you probably have read operations, > when you only want to write. That will depend on how much data you're trying to write. With 3 data discs and a 2M chunk size, writes in multiples of 6M won't need reads. Writing a 25M file would therefore write 4 stripes and need to read to do the last 1M. With 4 data discs, it'd be 8M multiples, and you'd write 3 stripes and need a read to do the last 1M. No difference. Cheers, John.