I've enabled compression (mounted with compress=lzo) for my btrfs partition and used it for a while.
I'm curious about how much benefit the compression brought me and am interested in the saved space value (sum of all file sizes) - (actual used space).
Is there any straightforward way to get this value, or would I have to write a script that sums up e.g. df output and compres it to btrfs filesystem dfoutput?
First install compsize. In Debian/Ubuntu:
apt install btrfs-compsize
In Fedora:
dnf install compsize
Then run it on your filesystem.
Add -x if you don't want to cross into other subvolumes or filesystems.
You must add -x if you have non-btrfs filesystems
mounted in the directory tree.
compsize /mnt/btrfs-partition
Example output:
Processed 123574 files, 1399139 regular extents (1399139 refs), 69614 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 73% 211G 289G 289G
none 100% 174G 174G 174G
lzo 32% 37G 115G 115G
It requires root (sudo) to work at all
(otherwise SEARCH_V2: Operation not permitted).
It can be used on any directory (totalling the subtree), not just the whole filesystem from the mountpoint.
On a system with zstd, but some old files still compressed with lzo,
there will be rows for each of them.
(The Perc column is the disk_size / uncompressed_size for that row,
not how much of the total is compressed that way. Smaller is better.)
Per the btrfs wiki, there's no built-in support for this, but the compsize tool can measure compression on btrfs.
The df output will be similar to btrfs fi df in that it will report how much disk space is used, not the uncompressed size. The wiki says you can "kind of guess" by comparing df output before and after compressing a file. Another option is using some program that will total up the actual data such as rsync --stats on a dry run, which will report how much data is read.
