From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brad Campbell Subject: Re: 3 disk raid-5 without parity Date: Mon, 14 Jun 2004 15:28:56 +0400 Sender: linux-raid-owner@vger.kernel.org Message-ID: <40CD8BF8.5040601@wasp.net.au> References: <20040614111920.GA17785@middle.of.nowhere> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20040614111920.GA17785@middle.of.nowhere> To: Jurriaan Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Jurriaan wrote: > I am trying to convince my boss our new database-server wants raid-0+1, > not raid-5, and I got an idea while reading endless articles about > raid-5 being slow when writing and management not listening. > > suppose you make a 3-disc raid-5 without parity: > > data disc1 disc2 disc3 > A A A B > B B C C > > How would that perform compared to raid-5 and raid-0+1? > > As I understand, the performance problem with raid-5 when writing is > that you may need to read old data to recompute the parity block, and > the write the parity block and the data in parallel. > > So in this case, you can read straight away from all 3 disks (with > raid-5 one of the disks will have parity information) and you can write > without reading old data. What you propose is not quite raid-5, but appears to be a striping hybrid between raid-0 and raid-1. If I get what you are saying, you are writing each block twice, and ensuring each block is written to at least 2 different drives. Interesting idea, but not really efficient. In addition, you are doubling your write bandwidth requirements (not unlike pure raid-1). Yes, raid-5 random write performance is limited by read-modify-write cycles when they are required. The raid-5 md driver does do a pretty good job of avoiding this where possible though. I have not really benchmarked my raid-5 write performance, but on reads I sit around 90MB/s across my 10 drives. (Even my SATA raid-0 with 2 7200RPM drives has trouble getting much quicker than this as I keep saturating the PCI bus) Regards, Brad