Readup

• Fedora's series on btrfs by Andreas Hartmann.
• Archived and obsolet sysadmin's guide to btrfs.
• Blog post on working with btrfs by Josphat Mutai with many examples.
• btrfs rtd.
• NixOS wiki entry on using btrfs with NixOS.

General

good.

COW

moo.

Subvols

Like directories, but can be manipulated like file systems, have snapshots and such. This stuff is kinda like bind mounts,

Commands:

• findmnt - display info about mounts.
• btrfs subvolume create <path> - create a subvol. path should be non existent.
• btrfs subvolume list <path> - show subvols in the fs containing path.
  • -o - show subvols below path.
• btrfs subvolume show <path> - show comprehensive information about subvol at path.
• rm -r <vol> or doas btrfs subvolume delete <vol> - delete subvol.
• umount <subvol> - unmount subvol after it has been mounted to a different path.
• mount -o subvol=<path>,<opts> <drive> <path> - mount subvol. subvol is how fstab knows where to mount what.

Options

• subvol - subvol path.
• subvolid - subvol id.
• u compression - subvol compression. See Compression.

Nested vs. flat

There are several basic schemas to layout subvolumes (and snapshots). $ Flat All subvols are children of root and later mounted into appropriate directories. E.g. you can create all your subvolumes in /subvolumes - a directory, with parent root, and use /snapshots - directory, - for snapshots.

$ Nested Subvols are located anywhere in file hierarchy - typically in their desired locations.

Comparison:

• management: easier with flat, because layout is directly visible and explicit.
• mounting: with flat all subvolumes need to be mounted separately, with nested no explicit mounting is needed.
• mount options: with flat options don't propagate from fs and need to be specified separately, but also can be modified per subvolume. With nested options propagate from fs.
• u visibility: ?
• snapshots: neither way will or include nested subvols in snapshots, so care should be taken. See snapshots.

Subvolume ID's

Displayed by doas btrfs subvolume list <path>. Start at 256 and increase by 1 for every new subvol. FS root always has subvol name / and id 5. When a btrfs partition is mounted without an explicit subvolume, subvolid=5 is assumed. list shows top level id, most often it's 5 - root.

Snapshots

Snapshots are a diff vcs on a fs level. $ Snapshot is a subvolume with added content: it holds references to current and/or past versions of files (inodes). See COW for how this is possible. Subvol snapshot won't contain children - so with subvols / and /home snapshoting / won't copy /home. Snapshots are created with btrfs subvolume snapshot <opts> <from> <to>. Benefit of snapshots over simple copying is COW - small changes to files will cause small changes to disk usage. This can be verified with compsize.

Backups

Storing snapshots on a single fs is convenient but not really secure - fs failure will cascade to snapshots. Remedying this requires efficiently exchanging information about snapshots between btrfs (or between btrfs and non-btrfs) file systems.

To back up a subvolume

• create a ro snapshot with btrfs subvolume snapshot -r <subvol> <snapshot>.
• transfer it with btrfs send <snapshot> | btrfs receive <dest>
• for non-btrfs destinations you can store snapshots in files - btrfs send -f <snapshot>.btrfs <snapshot>.

To utilize shared nodes between sent snapshots

• send incremental stream with btrfs send -p <parent> <snapshot>.

To restore a subvolume from a snapshot

• receive the snapshot with btrfs send <snapshot> | btrfs receive <dest>,
• recreate a rw subvol from a snapshot with btrfs subvolume snapshot <snapshot> <subvol>.

Compression

Can only be applied on a filesystem-wide level. blah blah.

Tools

• btrbk