Premature -ENOSPC on btrfs?

["Darrick J. Wong" <djwong@us.ibm.com>]

Hi,

I was giving btrfs (2.6.39-rc4) a quick tryout today and noticed some odd
behavior when running a rather stupid test.

First, I create a 1GB test fs, format it, and mount it.  Then, I run the
following command to create a huge file, truncate it, rewrite it, and report
what size file got created.

# while true; do dd if=/dev/zero of=/mnt/testfile bs=1024k; done

This is roughly what I see in terms of file size after filtering out all the
"dd: writing `/mnt/testfile': No space left on device" messages.

782237696 bytes (782 MB) copied, 16.5647 s, 47.2 MB/s
545259520 bytes (545 MB) copied, 14.061 s, 38.8 MB/s
780140544 bytes (780 MB) copied, 15.2959 s, 51.0 MB/s
933232640 bytes (933 MB) copied, 15.2241 s, 61.3 MB/s
827326464 bytes (827 MB) copied, 14.8383 s, 55.8 MB/s
931135488 bytes (931 MB) copied, 15.1554 s, 61.4 MB/s
936378368 bytes (936 MB) copied, 2.98301 s, 314 MB/s
393216000 bytes (393 MB) copied, 12.9202 s, 30.4 MB/s
387973120 bytes (388 MB) copied, 13.4906 s, 28.8 MB/s
932184064 bytes (932 MB) copied, 15.3356 s, 60.8 MB/s
785383424 bytes (785 MB) copied, 14.6429 s, 53.6 MB/s
927989760 bytes (928 MB) copied, 15.4386 s, 60.1 MB/s
833617920 bytes (834 MB) copied, 14.6289 s, 57.0 MB/s
936378368 bytes (936 MB) copied, 3.33651 s, 281 MB/s
393216000 bytes (393 MB) copied, 12.9689 s, 30.3 MB/s
389021696 bytes (389 MB) copied, 6.02794 s, 64.5 MB/s
294649856 bytes (295 MB) copied, 1.05144 s, 280 MB/s

Strangely, df reports that free space remains, and I can even create a second
file to fill the empty space, so clearly the fs isn't full yet.  Is this the
intended behavior of btrfs?  ext4/vfat seem to produce the same size file
every time.

--D