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Date: Sun, 10 Aug 2008 13:30:14 -0400 From: "Theodore Ts'o" <tytso@....edu> To: linux-ext4@...r.kernel.org Subject: Bug in delayed allocation: really bad block layouts! 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? - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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