From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from relay.sgi.com (relay1.corp.sgi.com [137.38.102.111]) by oss.sgi.com (Postfix) with ESMTP id C03B329DF8 for ; Sun, 30 Jun 2013 16:42:15 -0500 (CDT) Received: from cuda.sgi.com (cuda3.sgi.com [192.48.176.15]) by relay1.corp.sgi.com (Postfix) with ESMTP id AF70A8F8035 for ; Sun, 30 Jun 2013 14:42:11 -0700 (PDT) Received: from greer.hardwarefreak.com (mo-65-41-216-221.sta.embarqhsd.net [65.41.216.221]) by cuda.sgi.com with ESMTP id 5F2nyiFWIOkjZnmi for ; Sun, 30 Jun 2013 14:42:11 -0700 (PDT) Received: from [192.168.100.53] (gffx.hardwarefreak.com [192.168.100.53]) by greer.hardwarefreak.com (Postfix) with ESMTP id BB3826C054 for ; Sun, 30 Jun 2013 16:42:10 -0500 (CDT) Message-ID: <51D0A62E.2020309@hardwarefreak.com> Date: Sun, 30 Jun 2013 16:42:06 -0500 From: Stan Hoeppner MIME-Version: 1.0 Subject: Re: swidth in RAID References: <557F888F-34EA-4669-B861-C0B684DAD13D@gmail.com> In-Reply-To: <557F888F-34EA-4669-B861-C0B684DAD13D@gmail.com> Reply-To: stan@hardwarefreak.com List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: xfs-bounces@oss.sgi.com Sender: xfs-bounces@oss.sgi.com To: xfs@oss.sgi.com On 6/30/2013 1:43 PM, aurfalien wrote: > I understand swidth should = #data disks. No. "swidth" is a byte value specifying the number of 512 byte blocks in the data stripe. "sw" is #data disks. > And the docs say for RAID 6 of 8 disks, that means 6. > > But parity is distributed and you actually have 8 disks/spindles working for you and a bit of parity on each. > > So shouldn't swidth equal disks in raid when its concerning distributed parity raid? No. Lets try visual aids. Set 8 coffee cups (disk drives) on a table. Grab a bag of m&m's. Separate 24 blues (data) and 8 reds (parity). Drop a blue m&m in cups 1-6 and a red into 7-8. You just wrote one RAID stripe. Now drop a blue into cups 3-8 and a red in 1-2. Your second write, this time rotating two cups (drives) to the right. Now drop blues into 5-2 and reds into 3-4. You've written your third stripe, rotating by two cups (disks) again. This is pretty much how RAID6 works. Each time we wrote we dropped 8 m&m's into 8 cups, 6 blue (data chunks) and 2 red (parity chunks). Every RAID stripe you write will be constructed of 6 blues and 2 reds. XFS, or EXT4, or any filesystem, can only drop blues into the first 6 cups of a stripe. The RAID adds the two reds to every stripe. Maybe now you understand why sw=6 for an 8 drive RAID6. And now maybe you understand what "distributed parity" actually means--every stripe is shifted, not just the parity chunks. -- Stan _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs