From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from sandeen.net ([63.231.237.45]:37776 "EHLO sandeen.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726136AbeHAGZJ (ORCPT ); Wed, 1 Aug 2018 02:25:09 -0400 Subject: Re: agcount 33 by default for a single HDD? References: <20180801041204.GH2234@dastard> From: Eric Sandeen Message-ID: Date: Tue, 31 Jul 2018 23:41:30 -0500 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Chris Murphy , Dave Chinner Cc: xfs list On 7/31/18 11:38 PM, Chris Murphy wrote: > On Tue, Jul 31, 2018 at 10:12 PM, Dave Chinner wrote: >> On Tue, Jul 31, 2018 at 07:32:50PM -0600, Chris Murphy wrote: >>> This seems suboptimal. >> >> It's actually a very useful optimisation to make on thin devices. >> >>> Basically this is a 750G thin volume. I don't >>> have a plain partition on a device handy to try this out but I'm >>> pretty certain the default is 4 AG's in that case, so I'm confused why >>> by default 33 AGs are created on a thin volume. The LVM volume group >>> is on a dmcrypt PV. >> >> It's a thin volume, therefore it advertises an optimal IO size and >> alignment setting (i.e. the thin volume allocation chunk size). >> Hence mkfs.xfs treats it as a "multi-disk device" and sets up >> alignment and AG count appropriately. >> >> This is actually the right optimisation to make for sparse devices - >> more AGs increase filesystem concurrency but we normally restrict it >> on single spindles because each AG adds more seeks into typical >> workloads and slows them down. However, the thin volume already adds >> that penalty to the storage stack for us because they don't have a >> linear LBA-to-physical location characteristic. Hence we can >> increase filesystem concurrency without addition performance >> penalties being incurred. > > OK so why 33 AG's with xfsprogs 4.15, but 4 AG's with xfsprogs 4.17, > when directed to the same thin LV? And also the difference in sunit > and swidth? > Did you build 4.17 with --disable-blkid? -Eric