On Sun, Jul 22, 2012 at 10:55 PM, Swâmi Petaramesh [off-list ref] wrote:

You're painfully right Roman,

A freshly formatted 1 GB BTRFS filesystem on which 81 MB of data has been
put shows only ~260 MB of free space and reserves something like 2 x 380 MB
of metadata.

This is absolutely ridiculous of BTRFS... :-/

That's an artifact of the small size of that filesystem and the
default size of allocations, which is why mixed mode exists.

The metadata allocation is about 4% on most filesystems: I see 38gb of
allocated but unused metadata space on a 900gb fs and 70gb on a 1.7tb
fs, and the referenced threads reports 170gb on what appears to be a
4tb fs; while not ideal, it's not remotely as bad as the 25% overhead
of the minimum 256mb*2 metadata allocation on a small 1gb fs*.  The
behaviour of a small filesystem simply isn't the same as the behaviour
of a large filesystem.

* Note that 1gb is still considered a very rather btrfs filesystem,
for which mixed mode is recommended!