linux-ext4 - Re: mkfs.ext4 vs. e2fsck discard oddities

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Message-ID: <alpine.LFD.2.00.1202290801510.4926@dhcp-27-109.brq.redhat.com>
Date:	Wed, 29 Feb 2012 08:12:00 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
cc:	ext4 development <linux-ext4@...r.kernel.org>,
	Lukáš Czerner <lczerner@...hat.com>
Subject: Re: mkfs.ext4 vs. e2fsck discard oddities

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.

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
> 
> 

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