From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stan Hoeppner Subject: Re: RAID 6 reads all remaining chunks in a stripe when a single chunk is rewritten Date: Tue, 17 Dec 2013 07:02:29 -0600 Message-ID: <52B04B65.7070603@hardwarefreak.com> References: <52B02A08.4090007@informatik.uni-rostock.de> Reply-To: stan@hardwarefreak.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <52B02A08.4090007@informatik.uni-rostock.de> Sender: linux-raid-owner@vger.kernel.org To: Nikolaus Jeremic , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 12/17/2013 4:40 AM, Nikolaus Jeremic wrote: > Hi, > > I've did some Linux MD RAID 5 and 6 random write performance tests with > fio 2.1.2 (Flexible I/O tester) under Linux 3.12.4. However, the results > for RAID 6 show that writes to a single chunk in a stripe (chunk size is > 64 KB) result in more than 3 reads in case of more than 6 drives (tested > with 7, 8, and 9 drives) in the array (see fio statistics below). It > seems like that in the event of updating one data chunk in a stripe, all > of the remaining data chunks are read. > > By the way, in case of RAID 5 and 5 or more drives, the remaining chunks > seem not to be read when updating a single chunk in a stripe. > > Here is the fio job description: It would be easier and more deterministic if you'd simply use dd to write one full stripe, then seek to one chunk within that stripe and write one page. -- Stan