On Sun, Aug 10, 2008 at 01:30:14PM -0400, Theodore Ts'o wrote: > > Thanks to a comment on a recent blog entry of mine[1], I think I've > uncovered a rather embarassing bug in mballoc. > > [1]http://thunk.org/tytso/blog/2008/08/08/fast-ext4-fsck-times > > I created a fresh 5 gig ext4 filesystem, and then populating it using a > single-threaded tar command: > > (cd /usr ; tar cf - bin lib) | (cd /mnt; tar xfp -) > > I then unmounted the filesystem, and ran an instrumented e2fsck looking > for fragmented files, and found a whole series of fragmanted files with > the following pattern: > > Inode 122: (0):58399, (1-3):43703-43705 > Inode 124: (0):58400, (1):43707 > Inode 127: (0):58401, (1-7):43709-43715 > Inode 128: (0):58402, (1-2):43716-43717 > Inode 129: (0):58403, (1-3):43718-43720 > Inode 133: (0):58404, (1-5):43722-43726 > Inode 135: (0):58405, (1):43728 > Inode 136: (0):58406, (1-3):43729-43731 > Inode 141: (0-1):58407-58408, (2-6):43734-43738 > Inode 143: (0):58409, (1):43740 > Inode 144: (0):58410, (1-5):43741-43745 > Inode 146: (0):58411, (1):43746 > > Inode Pathname > 122 /bin/smproxy > 124 /bin/debconf-updatepo > 127 /bin/iostat > 128 /bin/xeyes > 129 /bin/pbmtog3 > 133 /bin/join-dctrl > 135 /bin/dpkg-name > 136 /bin/lockfile > 141 /bin/id > 143 /bin/ppmcolormask > 144 /bin/tty > 146 /bin/colrm > > If I do this test with -o nodelalloc, I get a slightly different > pattern. Now I get a whole series of discontiguous regions after the > first 15 blocks: > > inode last_block pblk lblk len > ============================================= > 2932: was 47087 actual extent 41894 (15, 3)... > 3512: was 47829 actual extent 41908 (15, 1)... > 3535: was 47904 actual extent 41912 (15, 37)... > 3549: was 47977 actual extent 41949 (15, 4)... > 3637: was 48225 actual extent 41959 (15, 6)... > 3641: was 48245 actual extent 41965 (15, 13)... > 3675: was 48418 actual extent 41978 (15, 1)... > 3675: was 41979 actual extent 48640 (16, 15)... > 3714: was 41984 actual extent 48656 (1, 2)... > 3954: was 49449 actual extent 48660 (15, 16)... > 3999: was 49569 actual extent 48679 (15, 2)... > 4010: was 49644 actual extent 48681 (15, 1)... > 4143: was 49943 actual extent 48687 (15, 10)... > 4202: was 50036 actual extent 48699 (15, 6)... > > So all of the discontiguities start at logical block #15, and when I > examine the inodes, what we find is one extent for blocks 0-14, ending > at the last_block number, and then the second extent which extends for > the rest of the file, starting somewhere else earlier in the block > group. > > So a very similar issue, even without delayed allocation. That leads me > to suspect the problem is somewhere inside mballoc. Aneesh, Andreas, > Alex --- I think you folks are most familiar the mballoc code the; > someone have time to take a look? This is clearly a bug, and clearly > something we want to fix. If we can't get an optimal layout with one > single-threaded process writing to the filesystem, what hope do we have > of getting it right on more realistic benchmarks or real-world usage? > Are these small files ? The locality group preallocation is discarded with the recent changes. 6be2ded1d7c51b39144b9f07d2c839e1bd8707f1 on linus tree ? -aneesh -- 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
