When data in a file is overwritten, starting somewhere in the middle of
the file, the overwritten space is counted twice against the space usage
numbers. Is this a bug, or did I something wrong?
This is what I did:
I create a subvolume and limit it to 4 MB, and create a 1000 KB file in
the subvol:
# btrfs subvol create s
Create subvolume './s'
# btrfs qgroup limit 4m s
# btrfs qgroup show ./ | grep 260
0/260 4096 4096
# dd if=/dev/zero of=s/file bs=1024 count=1000; sync
# ls -lah s/file
-rw-r--r--. 1 root root 1000K Apr 6 00:13 s/file
# btrfs qgroup show ./ | grep 260
0/260 1028096 1028096
Then I overwrite the last 900 KB of the file, and add 100 KB of data,
resulting in a 1.1 MB file. The space usage numbers shows 2 MB however:
# dd if=/dev/zero of=s/file bs=1024 count=1000 seek=100; sync
# ls -lah s/file
-rw-r--r--. 1 root root 1.1M Apr 6 00:13 s/file
# btrfs qgroup show ./ | grep 260
0/260 2052096 2052096
I repeat this twice, the file becomes 1.3 MB but the usage number goes
to almost 4 MB:
# dd if=/dev/zero of=s/file bs=1024 count=1000 seek=200; sync
# dd if=/dev/zero of=s/file bs=1024 count=1000 seek=300; sync
# ls -lah s/file
-rw-r--r--. 1 root root 1.3M Apr 6 00:14 s/file
# btrfs qgroup show ./ | grep 260
0/260 4100096 4100096
Doing the same again results in "quota exceeded" errors:
# dd if=/dev/zero of=s/file bs=1024 count=1000 seek=400; sync
dd: writing `s/file': Disk quota exceeded
78+0 records in
77+0 records out
78848 bytes (79 kB) copied, 0.00138135 s, 57.1 MB/s
# ls -lah s/file
-rw-r--r--. 1 root root 477K Apr 6 00:15 s/file
# btrfs qgroup show ./ | grep 260
0/260 4182016 4182016
# touch s/emptyfile
touch: cannot touch `s/emptyfile': Disk quota exceeded
Koen.
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