From: Mike Snitzer Subject: Re: Ext4 and xfs problems in dm-thin on allocation and discard Date: Tue, 19 Jun 2012 14:48:59 -0400 Message-ID: <20120619184858.GA8841@redhat.com> References: <4FDF9EBE.2030809@shiftmail.org> <20120619015745.GJ25389@dastard> <20120619031241.GA3884@redhat.com> <20120619131649.GA6811@redhat.com> <20120619133041.GB6811@redhat.com> <4FE0840F.2050704@shiftmail.org> <20120619144413.GA7225@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: =?utf-8?B?THVrw6HFoQ==?= Czerner , device-mapper development , linux-ext4@vger.kernel.org, Dave Chinner , xfs@oss.sgi.com To: Spelic Return-path: Received: from mx1.redhat.com ([209.132.183.28]:13074 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753101Ab2FSStL (ORCPT ); Tue, 19 Jun 2012 14:49:11 -0400 Content-Disposition: inline In-Reply-To: <20120619144413.GA7225@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, Jun 19 2012 at 10:44am -0400, Mike Snitzer wrote: > On Tue, Jun 19 2012 at 9:52am -0400, > Spelic wrote: > > > I do not know what is the mechanism for which xfs cannot unmap > > blocks from dm-thin, but it really can't. > > If anyone has dm-thin installed he can try. This is 100% > > reproducible for me. > > I was initially surprised by this considering the thinp-test-suite does > test a compilebench workload against xfs and ext4 using online discard > (-o discard). > > But I just modified that test to use a thin-pool with 'ignore_discard' > and the test still passed on both ext4 and xfs. > > So there is more work needed in the thinp-test-suite to use blktrace > hooks to verify that discards are occuring when the compilebench > generated files are removed. > > I'll work through that and report back. blktrace shows discards for both xfs and ext4. But in general xfs is issuing discards with much smaller extents than ext4 does, e.g.: to the thin device: + 128 vs + 32 to the thin-pool's data device: + 120 vs + 16