https://www.howtoforge.com/a-beginners-guide-to-btrfs
https://www.howtoforge.com/how-to-convert-an-ext3-ext4-root-file-system-to-btrfs-on-ubuntu-12.10
http://www.oracle.com/technetwork/articles/servers-storage-admin/advanced-btrfs-1734952.html

Compression

Create BTRFS filesystem on multiple block devices:

 mkfs.btrfs  /dev/loop0 /dev/loop1

btrfs filesystem show /dev/loop0
Label: none  uuid: e9a456cd-57d7-4355-a20a-77243d66722e
        Total devices 2 FS bytes used 28.00KiB
        devid    1 size 200.00MiB used 12.00MiB path /dev/loop0
        devid    2 size 200.00MiB used 0.00B path /dev/loop1

As you can see, every block belongs to FS can be used. Also for mount:

mount /dev/loop0 /BTRFS

or

mount /dev/loop1 /BTRFS

Current state: Do not create SWAP files on BTRFS!

Swap support was restored with Kernel 4.21

Add new block device (extend size)

 btrfs device add /dev/loop2 /BTRFS

Remove device:

btrfs device delete /dev/loop0 /BTRFS

!!! It is possible to remove any device if there is free space to which data can be moved during removal.

Start foreground, readonly scrub, print statistics at end:

btrfs scrub start -B -d -r /
 
scrub device /dev/sda3 (id 1) done
	scrub started at Mon Mar  6 07:02:56 2017 and finished after 00:04:06
	total bytes scrubbed: 18.50GiB with 0 errors
scrub device /dev/sdc5 (id 2) done
	scrub started at Mon Mar  6 07:02:56 2017 and finished after 00:06:52
	total bytes scrubbed: 18.47GiB with 0 errors

Get currently running scrub status:

btrfs scrub status /

apt-get install btrfs-tools

GRUB 2.02 doesn't support ZSTD compression. Solution is to exlude /boot from ZSTD compression (below) or install more recent Grub (from Debian Bullseye)

sudo btrfs filesystem defragment -v -r -czlib /boot
sudo btrfs property set /boot compression zlib
# disable compression for newly created files
chattr -Rc /boot

 31 #define ENOSPC          28      /* No space left on device */

# btrfs fi show /mount
Label: none  uuid: 5db83d5e-4f38-40b0-ac5d-dadd9cd1d23f
	Total devices 1 FS bytes used 22.18GiB
	devid    1 size 30.00GiB used 27.58GiB path /dev/mapper/vg_spox-lv_home
 
# btrfs fi usage /mount
Overall:
    Device size:		  30.00GiB
    Device allocated:		  27.58GiB
    Device unallocated:		   2.42GiB
    Device missing:		     0.00B
    Used:			  22.52GiB
    Free (estimated):		   5.58GiB	(min: 4.37GiB)
    Data ratio:			      1.00
    Metadata ratio:		      1.99
    Global reserve:		 512.00MiB	(used: 0.00B)
 
Data,single: Size:25.01GiB, Used:21.84GiB
   /dev/mapper/...	  25.01GiB
 
Metadata,single: Size:8.00MiB, Used:0.00B
   /dev/mapper/...	   8.00MiB
 
Metadata,DUP: Size:1.25GiB, Used:348.23MiB
   /dev/mapper/...	   2.50GiB
 
System,single: Size:4.00MiB, Used:0.00B
   /dev/mapper/...	   4.00MiB
 
System,DUP: Size:32.00MiB, Used:16.00KiB
   /dev/mapper/...	  64.00MiB
 
Unallocated:
   /dev/mapper/...	   2.42GiB

https://bbs.archlinux.org/viewtopic.php?id=200817 http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg60637.html