As commented on another post, I have done a backup of my main computer using rsync to another machine with a btrfs formatted hard disk of 8TB. Mounted with compression. Info: <https://btrfs.wiki.kernel.org/index.php/Compression> Isengard:~ # mount -v | grep btrfs /dev/mapper/cr_my_book_tlcntr on /mnt/BookTelcontar type btrfs (rw,relatime,lazytime,compress=zlib,space_cache,subvolid=5,subvol=/) Isengard:~ # Finally, I have found "compsize" - maybe I mistyped the name before: +++............... How can I determine compressed size of a file? compsize takes a list of files on a btrfs filesystem and measures used compression types and effective compression ratio: https://github.com/kilobyte/compsize There is a patch adding support fort that, currently it's not merged. You can kind of guess at its compressed size by comparing the output from the df command before and after writing a file, if this is available to you. ...............++- This is the report: Isengard:~ # time compsize --bytes /mnt/BookTelcontar/001/ Processed 3415745 files, 7871897 regular extents (8084571 refs), 1896373 inline. Type Perc Disk Usage Uncompressed Referenced TOTAL 97% 3687883543245 3783680719296 3879938206144 none 100% 3601384486854 3601384486854 3695579432902 zlib 47% 86499056391 182296232442 184358773242 real 33m58.873s user 0m47.452s sys 3m22.230s Isengard:~ # How do I interpret it? In particular, the "none" and "zlib" lines. This is another run on the main mail folder, which should compress a lot: Isengard:~ # time compsize --bytes /mnt/BookTelcontar/001/home1/cer/Mail Processed 5252 files, 120910 regular extents (120910 refs), 2043 inline. Type Perc Disk Usage Uncompressed Referenced TOTAL 60% 14290651991 23513871347 23513863155 none 100% 9204910666 9204910666 9204902474 zlib 35% 5085741325 14308960681 14308960681 real 0m1.726s user 0m0.135s sys 0m0.388s Isengard:~ # -- Cheers / Saludos, Carlos E. R. (from 15.0 x86_64 at Telcontar)