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

The answer for this is that btrbk is designed to never alter your
source subvolume. In the config above, the snapshots would be created
*inside* the source subvolume at: `/rootfs.20150101`.




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](https://btrfs.wiki.kernel.org/index.php/SysadminGuide) as
a good entry point.

<!-- TODO: add links to recommendations for ubuntu and other distros -->


## 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 ...