====== BTRFS as root filesystem ======

===== System overview =====
  - /dev/sda1 4.08GB ext4 "/" used 1.56GB, free 2.53GB
  - /dev/sda2 10.91GB extended
    - /dev/sda5 524MB swap
    - /dev/sda6 10.40GB ext4 "/home" 321MB used, 10.09GB free

===== Boot from live-cd =====
Btrfs support is heavily developed in Linux Kernel. So do not use old kernel.

I was using gparted-live-0.22.0-1-amd64.iso
(NOTE: to match your existing system architecture (i586, amd64, etc))
Which contains:
  - Kernel 3.16.0-4-586
  - btrfs utils version 3.17


===== Update to kernel 4 =====
Edit /etc/apt/sources and add sid release
<code bash>
apt-get update
apt-get install linux-image-amd64
apt-get install btrfs-tools
update-initramfs -u
</code>

  * ERROR: Warning: /sbin/fsck.btrfs doesn't exist, can't install to initramfs, ignoring.
  * EXPLANATION: "This is going to be changed back in the next version of btrfs-tools."
  * SOLUTION: ln -s /bin/fsck.btrfs /sbin/fsck.btrfs

Check btrfs-tools version
<code bash>/bin/btrfs version</code>


===== Converting dirs into subvolumes  =====
To make backups, snapshots, moving much easier and faster, every directory you want to backup by snapshot should be a subvolume. Also root filesystem should be created in separate subvolume not in btrfs root.

To distinguish volumes from directories we will use @ character at beginning of subvolume name. This is Ubuntu naming convention for subvolumes.

Proposed subvolumes structure (where / is BTRFS root)
  * /@
  * /@home
  * /@var

Mount BTRFS filesystem (BTRFS root) to /mnt/btrfs.

==== root ====
  * take a snapshot of root volume. Name of snapshot is '@'
    * ''btrfs subvolume snapshot /mnt/btrfs @''
  * delete files on BTRFS root (/mnt/btrfs)
  * make @ subvolume a default root
      * update fstab:<file | /etc/fstab>UUID=739e6086-d925-4bdb-94f5-26d8c10dc171	/ btrfs defaults,subvol=@</file>
    * OR 
      * get @ subvolume id ''btrfs subvolume list /mnt/btrfs''
      * set default subvolume to use when system is mounted ''btrfs subvolume set-default 256 /mnt/btrfs''


==== subdirectories ====
  * On BTRFS root create a new subvolume
    * ''btrfs subvolume create /mnt/btrfs/@home''
  * move files into the new subvolume as if it were a directory
    * ''mv /mnt/btrs/@/home/* /mnt/btrfs/@home/''
    * ''cp --reflink'' is much faster on BTRFS
  * remove old directory
    * ''rmdir /mnt/btrfs/@/home''
  * update /etc/fstab to mount @home as /home

===== fstab ====
Do not use ''errors=remount-ro'' option which is not valid for BTRFS.
Example working fstab file:
<file | /etc/fstab>
UUID=ed730508-753e-4a7f-ac1e-bf8646f7bd63	/		btrfs		defaults,subvol=@,compress=no				0	0
UUID=ed730508-753e-4a7f-ac1e-bf8646f7bd63	/var		btrfs		defaults,subvol=@var,compress=no			0	0
UUID=ed730508-753e-4a7f-ac1e-bf8646f7bd63	/var/log	btrfs		defaults,subvol=@varlog,compress=no			0	0
UUID=ed730508-753e-4a7f-ac1e-bf8646f7bd63	/home		btrfs		defaults,subvol=@home,compress=no			0	0
UUID=ed730508-753e-4a7f-ac1e-bf8646f7bd63	/mnt/btrfs	btrfs		noauto,defaults,subvolid=5,compress=no			0	0
</file>

To prevent systemd to halt system during startup if device is not found (nofail and x-systemd.device-timeout):
<file | /etc/fstab>
UUID=<the_device_uuid> /mount/point btrfs nofail,x-systemd.device-timeout=10,noatime,compress=lzo,autodefrag 0 0
</file>
NOTE: do not use autodefrag - it consumes lots of cpu