From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cuda.sgi.com (cuda1.sgi.com [192.48.157.11]) by oss.sgi.com (8.14.3/8.14.3/SuSE Linux 0.8) with ESMTP id p4V6BlJ5030833 for ; Tue, 31 May 2011 01:11:47 -0500 Received: from ipmail04.adl6.internode.on.net (localhost [127.0.0.1]) by cuda.sgi.com (Spam Firewall) with ESMTP id 5C99D15D9741 for ; Mon, 30 May 2011 23:11:44 -0700 (PDT) Received: from ipmail04.adl6.internode.on.net (ipmail04.adl6.internode.on.net [150.101.137.141]) by cuda.sgi.com with ESMTP id pjtdnx4E9XzxOSlI for ; Mon, 30 May 2011 23:11:44 -0700 (PDT) Date: Tue, 31 May 2011 16:11:42 +1000 From: Dave Chinner Subject: Re: Question about Number of Allocation Groups Message-ID: <20110531061142.GH561@dastard> References: <164529.98085.qm@web77701.mail.sg1.yahoo.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <164529.98085.qm@web77701.mail.sg1.yahoo.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 Sender: xfs-bounces@oss.sgi.com Errors-To: xfs-bounces@oss.sgi.com To: Gim Leong Chin Cc: xfs@oss.sgi.com On Tue, May 31, 2011 at 01:30:08PM +0800, Gim Leong Chin wrote: > Hi, > > I have a question about the number of allocation groups in XFS. > > In SUSE Linux Enterprise 10 SP2 kernel 2.6.16, the default number > of allocation groups from what I have seen is 16 for a 144 GB > filesystem, 32 for a 1.1 TB filesystem. > > In SUSE Linux Enterprise 11 kernel 2.6.27, the default number of > allocation groups is 4 even for a 800 GB filesystem (from what I > have observed on openSUSE 11.4 kernel 2.6.37, that number is still > 4). I have observed that for XFS on mdraid the default is 16 for > a 101 GB filesytem, 32 for a 4.5 TB filesystem. Yes, that sounds like the way the defaults currently work. > 1) May I know what is the method for computing the default number > of allocation groups, See the mkfs source code. It's described in the function calc_default_ag_geometry(). > and why the big change from kernel 2.6.16 to > 2.6.27 (16, 32 to 4)? Because disks are getting bigger. Defaults designed for disks of 10GB in size make no sense for current TB-sized disks.... > 2) What are the guide lines for deciding how many allocation > groups we should specify? Is the number related to filesystem > size, the IO bandwidth, the number of processor cores, the number > of parallelism threads we want to occupy the system, system > usage/workload characteristics? All of the above, and more. > 3) I am sure there is some point for any system where an > increasing number of allocation groups will first increase > performance, and then the performance will start to drop? Yes, there is. In general, increasing the # of AGs increases the inherent parallelism in the filesystem, but the tradeoff is that it will also increase the number of seeks workloads do. The optimum value is generally dependent on your storage hardware.... > 4) What would be an optimum number for the root filesystem for the > operating system? As I have 100 GB for the "/", my OS partitions > not on mdraid have the default 4 allocation groups for kernel > 2.6.27 and 2.6.37. The default, of course. Cheers, Dave. -- Dave Chinner david@fromorbit.com _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs