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I chose btrfs as the format of my / filesystem in the Ubuntu 12.04 LTS installer. After the installation completed, I added compress=lzo to the mount options in /etc/fstab and rebooted.

Will the existing files be automatically compressed now, or must I explicitly do something to cause that to happen?

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  • 1
    You don't have to reboot, you could just sudo mount -o remount / and the new fstab options will take effect.
    – sep332
    Jan 16, 2014 at 19:13
  • NB that you may need to use the compress-force option or some files will still remain uncompressed no matter what you do...
    – rogerdpack
    Mar 12, 2014 at 19:40
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    A note regarding compress-force. Normal compress has a feature in that if it detects that compression has little to no benefit, it doesn't bother continuing the compression for that particular file (thereby saving wasted CPU). By using compress-force, it attempts useless compression anyway. Good examples where this applies are for files that are already compressed (multimedia, zip files, etc). In other words, compress-force is generally a bad idea. ;)
    – zaTricky
    Oct 15, 2016 at 8:10

5 Answers 5

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31

You will have to run btrfs fi defragment to force recompression of existing data. Otherwise, only new data will be compressed.

From the FAQ:

...consider remounting with -o compress, and either rewrite particular files in-place, or run btrfs fi defragment to recompress everything. This may take a while.

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  • 1
    btrfs fi defragment -r -clzo /path/to/fs
    – David Goodwin
    Jul 30, 2015 at 14:25
  • 3
    What about the autodefrag mount option? Does that also make it compress previously uncompressed files when it auto-defrags?
    – Geremia
    Sep 20, 2016 at 14:02
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    Make sure to use "-r" the recursive flag for compressing within directories.
    – Salami
    Oct 14, 2016 at 17:30
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I've made what Norbert Fabritius said, but I didn't notice any compression in the existing files - df -h / before btrfs fi defragment = 658MB | df -h / after btrfs fi defragment = 658MB. New files are ok. Searching a little bring me this quote:

Running this:

# btrfs filesystem defragment ~/stuff

does not defragment the contents of the directory.

This is by design. btrfs fi defrag operates on the single filesystem object passed to >it. This means that the command defragments just the metadata held by the directory >object, and not the contents of the directory. If you want to defragment the contents >of the directory, something like this would be more useful:

# find -xdev -type f -exec btrfs fi defrag '{}' \;

After this, my / it's occupping 656MB - nothing huge, but certainly there is compression.

Source: https://btrfs.wiki.kernel.org/index.php/Problem_FAQ#Defragmenting_a_directory_doesn.27t_work

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  • 1
    1. When using btrfs, do not use or trust df. 2. For a better compression ratio, use zlib (however, it's slower)
    – ignis
    Nov 4, 2013 at 12:00
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    Shouldn't your command line include defrag -clzo or defrag -czlib instead of just defrag, if you want to actually compress things?
    – jbo5112
    Nov 17, 2013 at 5:14
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    There's a "-r" recursive flag so you can just do this now: btrfs fi defrag -r -czlib ~/stuff
    – Salami
    Oct 14, 2016 at 17:28
  • Just for reference, I got a 9.17 -> 5.89 GiB result with zstd:3 (default) on a fresh Ubuntu 22.04 install. Lovely.
    – martixy
    May 3 at 7:29
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According to Oracle's documentation, you can compress existing files on an existing, online filesystem by defragmenting each file in it with the -c, -clzo, or -czlib options. LZO is recommended for speed.

find / -xdev \( -type f -o -type d \) -exec btrfs filesystem defragment -v -clzo -- {} +

This uses the find command to run the btrfs defragmenter on every file in the root filesystem (given by the slash right after the "find" command at the beginning). If you have other subvolumes, you can use it again with the path of a subvolume (I have one at /home, for example) instead of the single slash.

You'll need root privileges for this, so add sudo to the front if you need to.

See:

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According to https://btrfs.wiki.kernel.org/index.php/Mount_options#List_of_options following compression algorithms are available:


compress,compress-force
Enable compression. Starting with kernel 2.6.38 you can choose the algorithm for compression:

 - compress=zlib - Better compression ratio. It's the default and safe for olders kernels.
 - compress=lzo - Faster compression.
 - compress=no - Disables compression (starting with kernel 3.6). 

compress-force= - Enable compression even for files that don't compress well, like videos and dd images of disks. The options compress-force=zlib and compress-force=lzo works for kernels >2.6.38.

Note that old (before 2012) btrfs-progs versions will probably fail some operations (e.g. fsck) on filesystems with LZO compression.

Note: This post is answer for the additional question what type of compression algorithms are available which was asked below...so please don't blame me.

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3

According to ArchWiki:

Tip: Compression can also be enabled per-file without using the compress mount option; simply apply chattr +c to the file. When applied to directories, it will cause new files to be automatically compressed as they come.

Very nice! God bless BTRFS!

Also, from the BTRFS wiki:

Can I force compression on a file without using the compress mount option?

Yes. The utility chattr supports setting file attribute c that marks the inode to compress newly written data.

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  • So the chattr method is for individual files while the btrfs filesystem defragment is for whole folders of files?
    – endolith
    Jan 23, 2020 at 22:17

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