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 > > -- -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
