sparse file handling bug in XFS

Sean Noonan
2011-06-16T14:49:24Z

Sparse files do not stay sparse.
Here's the simplest test case I've got so far. I don't think it can get much simpler than this.

This did not exist in 2.6.36. It appeared by 2.6.38-rc8. It continued into 2.6.38.2. It continues to exist on 3.0.0-rc3.

# for x in gogo xfs; do date | dd of=sparse-file bs=1k seek=4096; stat sparse-file; done
0+1 records in
0+1 records out
29 bytes (29 B) copied, 0.00151976 s, 19.1 kB/s
File: `sparse-file'
Size: 4194333 Blocks: 8 IO Block: 4096 regular file
Device: 6809h/26633d Inode: 12153368 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2011-06-14 13:56:43.826251881 +0000
Modify: 2011-06-14 13:56:43.826251881 +0000
Change: 2011-06-14 13:56:43.826251881 +0000
0+1 records in
0+1 records out
29 bytes (29 B) copied, 0.00152104 s, 19.1 kB/s
File: `sparse-file'
Size: 4194333 Blocks: 8192 IO Block: 4096 regular file
Device: 6809h/26633d Inode: 12153368 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2011-06-14 13:56:43.826251881 +0000
Modify: 2011-06-14 13:56:43.834252062 +0000
Change: 2011-06-14 13:56:43.834252062 +0000

For more fun, it even happens if we explicitly remove the file in between! (It looks like this is just because the recently freed inode is reused and not properly initialized.)

FILE=sparse-file-99; for x in gogo xfs; do rm -f $FILE; date | dd of=$FILE bs=1k seek=4096; stat $FILE; done
0+1 records in
0+1 records out
29 bytes (29 B) copied, 0.00151841 s, 19.1 kB/s
File: `sparse-file-99'
Size: 4194333 Blocks: 8 IO Block: 4096 regular file
Device: 6809h/26633d Inode: 12153371 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2011-06-14 14:03:37.933888145 +0000
Modify: 2011-06-14 14:03:37.937888543 +0000
Change: 2011-06-14 14:03:37.937888543 +0000
0+1 records in
0+1 records out
29 bytes (29 B) copied, 2.1681e-05 s, 1.3 MB/s
File: `sparse-file-99'
Size: 4194333 Blocks: 8192 IO Block: 4096 regular file
Device: 6809h/26633d Inode: 12153371 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2011-06-14 14:03:37.941888722 +0000
Modify: 2011-06-14 14:03:37.941888722 +0000
Change: 2011-06-14 14:03:37.941888722 +0000

Here is a run on ext3. Notice how blocks stays at 16.
# for x in gogo xfs; do date | dd of=sparse-file bs=1k seek=4096; stat sparse-file; done
0+1 records in
0+1 records out
29 bytes (29 B) copied, 0.00603038 s, 4.8 kB/s
File: `sparse-file'
Size: 4194333 Blocks: 16 IO Block: 4096 regular file
Device: 6805h/26629d Inode: 24 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2011-06-16 14:48:32.480750867 +0000
Modify: 2011-06-16 14:48:32.480750867 +0000
Change: 2011-06-16 14:48:32.480750867 +0000
0+1 records in
0+1 records out
29 bytes (29 B) copied, 0.00125449 s, 23.1 kB/s
File: `sparse-file'
Size: 4194333 Blocks: 16 IO Block: 4096 regular file
Device: 6805h/26629d Inode: 24 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2011-06-16 14:48:32.480750867 +0000
Modify: 2011-06-16 14:48:32.492751347 +0000
Change: 2011-06-16 14:48:32.492751347 +0000