OK, so with df I get
root@CUBE:~# du -sh /mnt/disk1
304G /mnt/disk1
root@CUBE:~# df -h /mnt/disk1
Filesystem Size Used Avail Use% Mounted on
/dev/md1 3.7T 291G 3.4T 8% /mnt/disk1
I managed to compile compsize, and it shows me this
root@CUBE:~# /root/compsize/compsize /mnt/disk1
Processed 5060 files, 1075817 regular extents (1075817 refs), 176 inline.
Type Perc Disk Usage Uncompressed Referenced
TOTAL 95% 289G 303G 303G
none 100% 226G 226G 226G
zlib 81% 63G 77G 77G
I compiled compsize like this:
somewhere (I was in /root) I cloned https://github.com/kilobyte/compsize
I installed the Dev Pack plugin, and installed all the packages inside (you could probably cherry picking them)
I downloaded btrfs-progs (the one already installed had no headers) from (see note in red at the end of this post) https://slackware.pkgs.org/14.2/slackware-i486/btrfs-progs-v4.5.3-i586-1.txz.html
I upgraded btrfs-progs with upgradepkg --install-new btrfs-progs-v4.5.3-i586-1.txz (upgradepkg reported: btrfs-progs-v4.15.1-x86_64-1 upgraded with new package ./btrfs-progs-v4.5.3-i586-1.txz)
In the cloned compsize repo, I ran make and got by that the compsize binary in there
Note: I'm on unRAID 6.5.3 (as per /etc/unraid-version) and apparently that's Slackware 14.2 (as per /etc/slackware-version), this version should match in the URL of the package you download.
And I just noticed I installed from slackware-i486 instead of slackware-x86_64, upgraded now to the one from https://slackware.pkgs.org/14.2/slackware-x86_64/btrfs-progs-v4.5.3-x86_64-1.txz.html for consistency (and safety I guess); I recompiled compsize and the report on compression is the same.