this post was submitted on 06 May 2024
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It is only for new data.
For example, you would have to defragment your filesystem again with
btrfs filesystem defragment -r -v -czstd /
. Wherezstd
is an algorithm and/
, a root path. With this command, the default compression level will be used, which is level 3.Be careful, defragmenting the btrfs file system will/can duplicate the data.
As for a mount point, if you decided to use zstd algorithm with level 1 compression, just add the
compress=zstd:1
orcompress-force=zstd:1
to the mount options (fstab or while mounting manually)Reading the manpage (btrfs-filesystem), duplication can happen on some odd kernel versions, so no danger.
Edit: that was my interpretation of breaking up reflinks of cow data anyway. Seems there's more.
If I know correctly, defrag will always duplicate the reflink files.
https://btrfs.readthedocs.io/en/latest/Defragmentation.html
Well, compression doubled my available space. ;-)
So I set up my system with btrfs in the last days and I converted two external drives (from ext4) (mainly game) and run defrag and balance, because it was mentioned in a guide to compress the existing files. Was that a bad idea? Didn't read anything about duplicates.
It is fine. You can use the duperemove tool (or bees) to find and remove duplicates.
https://btrfs.readthedocs.io/en/latest/Deduplication.html
So it is out-of-band deduplication and has to be done manually.
Also, by default cp and most file managers use a reflink copy (data blocks are copied only when modified)