BTrFS crashhhh?

Solution 1:

From the sounds of it, you're running into this:

Allocation is done on a round-robin basis. If you have a raid1 strategy on a volume made up of mismatched drives (volumes of differing sizes), your smaller volume may fill up while leaving lots of space free on your single largest drive. You can verify that this is an issue if there is any discrepancy between 'df' and 'btrfs filesystem df [mountpoint]' AND if the latter command also shows that "total" and "used" are the same on the "Data" line. A rebalance may mitigate this problem. (2.6.33)

• If your volume does fill up in this manner, a rebalance may quickly cause an ENOSPC ("Error NO SPaCe left on device") oops. You may have to delete a relatively large file to resolve this impasse, then a rebalance will succeed. (2.6.33)

https://btrfs.wiki.kernel.org/index.php/Gotchas

Emphassis mine. The second partition on a third 80GB drive you mention is probably filling up well before the pair of 250GB drives are, and it's triggering this particular Gotcha.

Also, BTrFS is a beta filesystem for a reason.

Solution 2:

I would refer you to: http://packages.ubuntu.com/natty/btrfs-tools which states:

WARNING: Btrfs is under heavy development, and is not suitable for any uses other than benchmarking and review.

I doubt you're going to get much help outside of the mailing list.