compsize takes a list of files (given as arguments) on a btrfs filesystem and measures used compression types and effective compression ratio, producing a report such as:

[~]$ compsize /home
Processed 140058 files, 133128 regular extents (196786 refs), 80886 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL       93%       14G          15G          12G
none       100%       13G          13G          10G
zlib        41%      628M         1.4G         1.4G
zstd        28%       42M         148M         148M

A directory has no extents but has a (recursive) list of files. A non-regular file is silently ignored.

As it makes no sense to talk about compression ratio of a partial extent, every referenced extent is counted whole, exactly once -- no matter if you use only a few bytes of a 1GB extent or reflink it a thousand times. Thus, the uncompressed size will not match the number given by tar or du. On the other hand, the space used should be accurate (although obviously it can be shared with files outside our set).

The fields are:

• Type: compression algorithm used
• Perc: disk usage/uncompressed -- ie, effective compression ratio
• Disk Usage: blocks actually used on the disk
• Uncompressed: extents before compression
• Referenced: apparent size of files (minus holes)

The ioctl used requires root.

Besides regular C toolchain, you need btrfs userspace headers. On Debian (incl. derivatives like Ubuntu) they're in libbtrfs-dev, SuSE ships them inside libbtrfs-devel, they used to come with btrfs-progs before. Required kernel: 3.16, btrfs-progs: 3.18 (untested!).