I was trying some various IO patterns to see what the ext4 allocator might do (as I tend to do every few months ....) :) On the one hand there are some very interesting, and nice (at least for some workloads) results: If I write even, then odd, blocks, in the end it comes out to one extent - even with an unmount in between: # for I in `seq 0 2 1024`; do dd if=/dev/zero of=testfile bs=4k count=1 conv=notrunc seek=$I 2>/dev/null; done (unmount, remount) # for I in `seq 1 2 1024`; do dd if=/dev/zero of=testfile bs=4k count=1 conv=notrunc seek=$I 2>/dev/null; done # filefrag testfile File is stored in extents format testfile: 1 extent found as long as the holes eventually get filled in, this is a pretty nice behavior to end up with contiguous allocation (if they're not ever filled in, it's a little odd) However, sequential, synchronous writes are doing weird things: # for I in `seq 1 1024`; do dd if=/dev/zero of=testfile bs=4k count=1 conv=notrunc seek=$I oflag=sync 2>/dev/null; done # filefrag -v testfile Checking testfile Filesystem type is: ef53 Filesystem cylinder groups is approximately 235 File is stored in extents format Blocksize of file testfile2 is 4096 File size of testfile2 is 4198400 (1025 blocks) First block: 0 Last block: 45312 Discontinuity: Block 2 is at 44032 (was 43520) Discontinuity: Block 11 is at 43521 (was 44040) Discontinuity: Block 15 is at 43066 (was 43524) Discontinuity: Block 256 is at 44544 (was 43306) testfile: 5 extents found not only is it non-contiguous, it's out of order. Ditto for direct IO: # for I in `seq 1 1024`; do dd if=/dev/zero of=testfile bs=4k count=1 conv=notrunc seek=$I oflag=direct 2>/dev/null; done [root@inode test2]# filefrag -v testfile Checking testfile Filesystem type is: ef53 Filesystem cylinder groups is approximately 235 File is stored in extents format Blocksize of file testfile is 4096 File size of testfile is 4198400 (1025 blocks) First block: 0 Last block: 45312 Discontinuity: Block 2 is at 43525 (was 44041) Discontinuity: Block 4 is at 44042 (was 43526) Discontinuity: Block 5 is at 43527 (was 44042) Discontinuity: Block 15 is at 43306 (was 43536) Discontinuity: Block 16 is at 43312 (was 43306) Discontinuity: Block 128 is at 43136 (was 43423) Discontinuity: Block 256 is at 44544 (was 43263) testfile: 8 extents found Interestingly, a backwards synchronous write comes out exactly the same: [root@inode test2]# for I in `seq 1024 -1 0`; do dd if=/dev/zero of=testfile2 bs=4k count=1 conv=notrunc seek=$I oflag=sync 2>/dev/null; done [root@inode test2]# filefrag -v testfileChecking testfile Filesystem type is: ef53 Filesystem cylinder groups is approximately 235 File is stored in extents format Blocksize of file testfile is 4096 File size of testfile is 4198400 (1025 blocks) First block: 0 Last block: 45312 Discontinuity: Block 2 is at 43525 (was 44041) Discontinuity: Block 4 is at 44042 (was 43526) Discontinuity: Block 5 is at 43527 (was 44042) Discontinuity: Block 15 is at 43306 (was 43536) Discontinuity: Block 16 is at 43312 (was 43306) Discontinuity: Block 128 is at 43136 (was 43423) Discontinuity: Block 256 is at 44544 (was 43263) testfile: 8 extents found It's not an artifact of filefrag; debugfs shows it as well: BLOCKS: (IND):43066, (1):44041, (2-3):43525-43526, (4):44042, (5-14):43527-43536, (15):43306, (16-127):43312-43423, (128-255):43136-43263, (256-1024):44544-45312 not sure what's going on here, have not started digging yet, but it's .... odd. With delallloc and buffered (non-synchronous IO), these all come out pretty sanely. -Eric -- 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
