On 02/25/2014 09:01 PM, thanumalayan mad wrote:
Hi all,
Slightly complicated question.
Assume I do two directory operations in a Btrfs partition (such as an
unlink() and a rename()), one after the other, and a crash happens
after the rename(). Can Btrfs (the current version) send the second
operation to the disk first, so that after the crash, I observe the
effects of rename() but not the effects of the unlink()?
I think I am observing Btrfs re-ordering an unlink() and a rename(),
and I just want to confirm that my observation is true. Also, if Btrfs
does send directory operations to disk out of order, is there some
limitation on this? Like, is this restricted to only unlink() and
rename()?
I am looking at some (buggy) applications that use Btrfs, and this
behavior seems to affect them.
There isn't a single answer for this one.
You might have
Thread A:
ulink(foo);
rename(somefile, somefile2);
<crash>
This should always have the rename happen before or in the same
transaction as the rename.
Thread A:
ulink(dirA/foo);
rename(dirB/somefile, dirB/somefile2);
Here you're at the mercy of what is happening in dirB. If someone
fsyncs that directory, it may hit the disk before the unlink.
Thread A:
ulink(foo);
rename(somefile, somefile2);
fsync(somefile);
This one is even fuzzier. Backrefs allow us to do some file fsyncs
without touching the directory, making it possible the unlink will hit
disk after the fsync.
-chris
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