BTRFS and power loss ~= corruption?

[Berend Dekens <btrfs@cyberwizzard.nl>]

Hi,

I have followed the progress made in the btrfs filesystem over time and 
while I have experimented with it a little in a VM, I have not yet used 
it in a production machine.

While the lack of a complete fsck was a major issue (I read the update 
that the first working version is about to be released) I am still 
worried about an issue I see popping up.

How is it possible that a copy-on-write filesystem becomes corrupted if 
a power failure occurs? I assume this means that even (hard) resetting a 
computer can result in a corrupt filesystem.

I thought the idea of COW was that whatever happens, you can always 
mount in a semi-consistent state?

As far as I can see, you wind up with this:
- No outstanding writes when power down
- File write complete, tree structure is updated. Since everything is 
hashed and duplicated, unless the update propagates to the highest 
level, the write will simply disappear upon failure. While this might be 
rectified with a fsck, there should be no problems mounting the 
filesystem (read-only if need be)
- Writes are not completed on all disks/partitions at the same time. The 
checksums will detect these errors and once again, the write disappears 
unless it is salvaged by a fsck.

Am I missing something? How come there seem to be plenty people with a 
corrupt btrfs after a power failure? And why haven't I experienced 
similar issues where a filesystem becomes unmountable with say NTFS or 
Ext3/4?

-- 
Regards,
Berend Dekens