On 2/29/12 1:12 AM, Lukas Czerner wrote:

On Tue, 28 Feb 2012, Eric Sandeen wrote:

quoted

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

quoted

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