From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Sandeen Subject: Re: mkfs.ext4 vs. e2fsck discard oddities Date: Wed, 29 Feb 2012 10:01:53 -0600 Message-ID: <4F4E4BF1.1060503@redhat.com> References: <4F4D1020.5060204@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: ext4 development To: Lukas Czerner Return-path: Received: from mx1.redhat.com ([209.132.183.28]:49002 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758083Ab2B2QBz (ORCPT ); Wed, 29 Feb 2012 11:01:55 -0500 Received: from int-mx12.intmail.prod.int.phx2.redhat.com (int-mx12.intmail.prod.int.phx2.redhat.com [10.5.11.25]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id q1TG1tWg008863 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 29 Feb 2012 11:01:55 -0500 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On 2/29/12 1:12 AM, Lukas Czerner wrote: > On Tue, 28 Feb 2012, Eric Sandeen wrote: > >> I've been testing Lukas' last 2 patches for e2fsck discard, and noticed something a little odd. >> >> If I make a 512M file, loopback mount it, and mkfs.ext4 it with discard, it uses about 17M at that point. >> If I then run fsstress on it with a known seed, then run e2fsck -E discard on it, it uses about 52M. >> >> If I repeat the above test telling mkfs.ext4 NOT to discard, I'm left with about 94M after the discarding e2fsck. >> >> So it seems that perhaps e2fsck is not discarding everything that it could; after a discarding fsck, we should be left with the same (minimal) nr. of blocks "in use" no? > > The reason is (as I commented in the patch #2) that we will not discard > BLOCK_UNINIT groups. We use BLOCK_UNINIT as a optimization measure to > skip groups which are likely to be non-provisioned, because we have > never written there anything since the mkfs. > > If you create file system without discard, then obviously nothing is > discarded, image is fully provisioned and e2fsck discard *only* initialized > groups. So you'll end up with the bigger image, in case that your image was > not sparse. > > I hope that makes sense. It does, sorry, I had been focusing too much on patch #1 ;) -Eric > Actually I want to make the same optimization for fitrim. We discussed > it with Ted and Phillip (see the discussion under [RESEND] [PATCH 2/2 > v2] e2fsck: Do not forget to discard last block group. They did seem to > be convinced by that, however I think it is right thing to do for the > reasons I gave in that thread. > > Thanks! > -Lukas > >> >> I guess that's better than discarding _more_ than it should though. ;) >> >> (I suppose it is possible that this is the underlying filesytem being selective about which discards it accepts, but it behaves the same way on ext4 and xfs backing filesystems) >> >> -Eric >> >> FWIW, sequence of events here, tested with and without "-K" on mkfs.ext4: >> >> dd if=/dev/zero of=fsfile bs=1M count=512 >> losetup /dev/loop0 fsfile >> mkfs.ext4 -F /dev/loop0&>/dev/null >> mount /dev/loop0 mnt/ >> /root/git/xfstests/ltp/fsstress -s 1 -d mnt/ -n 2000 -p 4 >> umount mnt/ >> e2fsck/e2fsck.static -fy -E discard /dev/loop0> fsck1.out || exit >> du -hc fsfile >> losetup -d /dev/loop0 >> >> >