On Fri, Sep 25, 2020 at 8:22 AM Matti Pulkkinen <mkjpul(a)utu.fi> wrote:
Is there a way in the Fedora 32 installer to set the compression mount
option for a new BTRFS filesystem before installing Fedora itself?
If you're familiar with kickstart, you can use '--fsoptions
compress=zstd:1' This will use this option for installation and also
add it to fstab.
Another option is to start the installation, switch to Terminal, and
'mount -o remount,compress=zstd:1'. If you're fast, you'll beat the
mkfs and initial mount, and get an error. Just up arrow and reissue
the command until it works. It doesn't really have to be exact - Btrfs
compression uses a maximum extent size of 128KiB. It's routine for
files to have mixed compression for blocks.
know I can add the option to the fstab file after installation,
and then run the defragment command to compress after the fact, but it
would be convenient if I could just have the filesystem compressed right
away during installation.
It's fine to just add to fstab and continue on without defragmenting.
As updates happen, most everything becomes compressed through
Also, from what I've read I think I already know the answer to
I'll ask to make sure: is it possible to set the compression option
persistently in the filesystem itself?
You can set a compression xattr per subvolume, directory and file.
Inheritance works like any other xattr.
There's a couple of ways to set it. The more familiar method is
'chattr +c' not to be confused with '+C' for nodatacow. Yes, it's a
file attribute but it actually sets an xattr. The default compression
algorithm is still zlib. So that's what you get with this method. If
you want to use zstd or lzo, you'll need to use e.g.
# btrfs property set -ti myfiles compression zstd
# btrfs property get myfiles
This uses the default zstd level of 3. Currently it isn't
configurable, but there are patches upstream to do that eventually.
It would be nice if I could have
an external USB hard drive (with its obvious IO bottleneck) compressed
as well. However, it seems like I would then have to always manually
mount the drive with the compression option, or set the compression
attribute after mounting. Either way, this isn't persistent. I also know
I could set the disk's UUID to always be mounted with the compression
option, but that is specific to one machine.
This should work, but I haven't tested it:
* create a subvolume in the top-level  of the file system
* set the compression xattr on that subvolume per either method
* use 'btrfs subvolume set-default /mnt/subvolume'
So now when you mount this file system without any options, it'll
mount this subvolume rather than the top-level, and it's set to
If for whatever reason you need to mount the top-level again, you'll
do that with e.g. 'mount -o subvol=/ /dev/sdXY /mnt'
The top-level is jargon for the permanent, unnamed, invisible
subvolume, created at mkfs time. It's ID is 0. The original default
subvolume created at mkfs time has an ID of 5 for historical reasons,
and later on an alias of 0 was created. Therefore its subvol name is
just / as I used above; or you can use subvolid=0 or subvolid=5. So
yeah, three ways to do the same thing. Whichever you're able to
remember easier, they're all the same.)