On 05/17/2010 02:04 PM, Jakob Unterwurzacher wrote:
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.
Nearly does not seem that reassuring. What would happen if the server
was under an intense load, swapping away crazily and running multiple
writers to that same file system?
ric
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
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