Hi!

Following Ubuntu's dpkg+ext4 problems I wanted to see if btrfs would
solve them all. And it nearly does! Now I wonder if the remaining 0.2
seconds window of exposing 0-size files could be closed too.

I tested using two simple scripts (attached for reference) on kernel
2.6.34-rc7:
- rentest creates files $i.tmp and renames to $i.cur,
- owtest does the same but overwrites existing $i.cur files,
letting them run for 30-50 seconds then resetting the virtual machine.

The results for ext3 are as expected: 0-size files are never exposed as
$i.cur, overwrites are atomic.

ext4 overwrites are /almost/ atomic (I get one 0-size file in owtest),
lots of 0-size files are exposed in rentest (30 seconds window).

btrfs *nearly* does as well as ext3. Overwrites are atomic.

The rentest exposes only a 0.2 seconds windows of 0-size $i.cur files,
so that a "ls --full-time" after the crash looks like this (notice the
time between 01281.cur and 01292.tmp, only 0.2 seconds):
[...]
-rw-r--r-- 1 root root 20 2010-05-17 17:06:25.812016407 +0200 01280.cur
-rw-r--r-- 1 root root 20 2010-05-17 17:06:25.835999490 +0200 01281.cur
-rw-r--r-- 1 root root  0 2010-05-17 17:06:25.868035485 +0200 01282.cur
[...]
-rw-r--r-- 1 root root  0 2010-05-17 17:06:26.080003626 +0200 01291.cur
-rw-rw-rw- 1 root root  0 2010-05-17 17:06:26.108010083 +0200 01292.tmp


Finally, xfs kills lots of existing files in owtest and exposes lots of
0-size files in rentest (both 40 seconds window).

If anybody is interested, the bunch of trimmed "ls --full-time" output
for all filesystems is attached.


Thanks,
Jakob