btrbk FAQ

Why is it not possible to backup '/' (btrfs root) ?

or in other words: why does this config not work:

/etc/btrbk/btrbk.conf:

volume /
  subvolume /
    snapshot_name rootfs

ERROR: Only relative files allowed for option "subvolume".

Answer

btrbk is designed to never alter your source subvolume. In the config above, the btrbk snapshots would be created inside the source subvolume, altering it.

The same applies to any "btrfs root" mount point (subvolid=0). In the example below, you will not be able to backup /mnt/data using btrbk:

/etc/fstab:

/dev/sda1  /mnt/data  btrfs  subvolid=0 [...]

btrbk is designed to operate on the subvolumes within /mnt/data. The recommended way is to split your data into subvolumes, e.g.:

 # btrfs subvolume create /mnt/data/www
 # btrfs subvolume create /mnt/data/mysql
 # btrfs subvolume create /mnt/data/projectx

This way you make full advantage of the btrfs filesystem, as all your data now has a name, which helps organizing things a lot. This gets even more important as soon as you start snapshotting and send/receiving.

The btrbk configuration for this would be:

volume /mnt/data
  subvolume www
    [...]
  subvolume mysql
    [...]
  subvolume projectx
    [...]

Tech Answer

While btrfs root (subvolid=0) is a regular subvolume, it is still special: being the root node, it does not have a "name" inside the subvolume tree.

Here, /mnt/btr_pool is mounted with subvolid=0:

# btrfs sub show /mnt/btr_pool/
/mnt/btr_pool is btrfs root

# btrfs sub show /mnt/btr_pool/rootfs
/mnt/btr_pool/rootfs
        Name:    rootfs
        uuid:    [...]

How should I organize my btrfs filesystem?

There's lots of ways to do this, and each one of them has its reason to exist. Make sure to read the btrfs SysadminGuide on kernel.org as a good entry point.

btrfs root

If your linux root filesystem is btrfs, I recommend booting linux from a btrfs subvolume, and use the btrfs root only as a container for subvolumes (i.e. NOT booting from "subvolid=0"). This has the big advantage that you can choose the subvolume in which to boot by simply switching the rootflags=subvol=<subvolume> kernel boot option.

Example (/boot/grub/grub.cfg):

menuentry 'Linux' {
  linux /boot/vmlinuz root=/dev/sdb3 ro rootflags=subvol=rootfs quiet
}
menuentry 'Linux (testing)' {
  linux /boot/vmlinuz root=/dev/sdb3 ro rootflags=subvol=rootfs_testing
}

Note that btrbk snapshots and backups are read-only, this means you have to create a run-time (rw) snapshot before booting into it:

# btrfs subvolume snapshot /mnt/btr_pool/backup/btrbk/rootfs-20150101 /mnt/btr_pool/rootfs_testing

How do I convert '/' (subvolid=0) into a subvolume?

There's several ways to achieve this, the solution described below is that it guarantees not to create new files (extents) on disk.

Step 1: make a snapshot of your root filesystem

Assuming that '/' is mounted with subvolid=0:

# btrfs subvolume snapshot / /rootfs

Note that this command does NOT make any physical copy of the files of your subvolumes within "/", it will only add some metadata.

Step 2: make sure that "/rootfs/etc/fstab" is ok.

Add mount point for subvolid=0 to fstab, something like this:

/rootfs/etc/fstab:

/dev/sda1  /mnt/btr_pool  btrfs  subvolid=0,noatime  0 0

Step 3: boot from the new subvolume "rootfs".

Either add rootflags=subvol=rootfs to grub.cfg, or set subvolume "rootfs" as default:

# btrfs subvolume set-default <subvolid> /

Step 4: after reboot, check if everything went fine:

First check your system log for btrfs errors, then:

# btrfs subvolume show /
        Name:                   rootfs
        ...

Great, this tells us that we just booted into our new snapshot!

# mount /mnt/btr_pool
# btrfs subvolume show /mnt/btr_pool
/mnt/btr_pool is btrfs root

This means that the root volume (subvolid=0) is correctly mounted.

Step 5: delete old (duplicate) files

Carefully delete all old files from /mnt/btr_pool, except "rootfs" and all other subvolumes within "/". You can list all these by typing:

# btrfs subvolume list -a /mnt/btr_pool

Make sure you do NOT delete anything within the directories listed here!

something like:

# cd /mnt/btr_pool
# rm -rf bin sbin usr lib var ...