This is a walkthrough for creating btrfs subvolumes. It was originally a larger document detailing the installation of Arch Labs from formatting the hdd all the way up to verification, mounting and swapon. I'm only concerned about using Step 5's info for creating subvolumes via chrooting from another Arch or Manjaro liveUSB. 

archlabs-btrfs

Install ArchLabs <www.archlabslinux.com> 2018.03 with Btrfs filesystem.

Disk Partition Layout

1. boot (512M) with ext4 filesystem,
2. home (5G) with xfs filesystem,
3. root (5G) with btrfs (@, @btrfs, @pkg, @log, @snapshots),
4. swap (2.5G) with swap space.

For demonstration purposes only. Do not use this scheme for any live or productive system.

Preparations

1. Create a full backup of your current system (!).
2. Download ArchLabs Linux from here and save it.
3. Copy ISO image to USB stick with dd: dd if=archlabs-2018-03.iso of=/dev/sdX bs=5M && sync
4. Boot ArchLabs (x86_64) with your fresh USB stick.
5. Right-Click on Desktop and select "Install Archlabs".

System Setup

1. Select Language.

2. Go to Prepare System.

   1. Set Keyboard Layout.
   2. Partition Device: /dev/sda 13G
   3. Chose cfdisk and select gpt for label type.
   4. [NEW] -> Partition size: 512M # Boot
   5. [NEW] -> Partition size: 5G # Root with @, @pkg, @log, @snapshots
   6. [NEW] -> Partition size: 5G # Home
   7. [NEW] -> Partition size: 2.5G # Swap
   8. Click on Write, confirm and exit.

root@ArchLabs / # fdisk -l
Disk /dev/sda: 13 GiB, 13958643712 bytes, 27262976 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: D696C02E-4788-2346-AA7A-C8E23E1FCDE7

Device        Start      End  Sectors  Size Type
/dev/sda1      2048  1050623  1048576  512M Linux filesystem
/dev/sda2   1050624 11536383 10485760    5G Linux filesystem
/dev/sda3  11536384 22022143 10485760    5G Linux filesystem
/dev/sda4  22022144 27262942  5240799  2.5G Linux filesystem

...

3. Click CTRL+F2 and login as root with password archlabs.
4. Format partitions:
   1. mkfs.ext4 -L boot.ext4 /dev/sda1 # Boot
   2. mkfs.btrfs -L root.btrfs /dev/sda2 # Root with @, @pkg, @log, @snapshots
   3. mkfs.xfs -L home.xfs /dev/sda3 # Home
   4. mkswap -L swap.swap /dev/sda4 # Swap

root@ArchLabs / # mkfs.ext4 -L boot.ext4 /dev/sda1
mke2fs 1.43.9 (8-Feb-2018)
Creating filesystem with 131072 4k blocks and 32768 inodes
Filesystem UUID: 2de35468-684c-4804-a29c-51aa708bf5e0
Superblock backups stored on blocks:
       32768, 98304

Allocating group tables: done
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done


root@ArchLabs / # mkfs.btrfs -L root.btrfs /dev/sda2
btrfs-progs v4.15.1
See http://btrfs.wiki.kernel.org for more information.

Label:              root.btrfs
UUID:               0fdbc292-a9b2-4862-a4be-61b019db243a
Node size:          16384
Sector size:        4096
Filesystem size:    5.00GiB
Block group profiles:
 Data:             single            8.00MiB
 Metadata:         DUP             256.00MiB
 System:           DUP               8.00MiB
SSD detected:       no
Incompat features:  extref, skinny-metadata
Number of devices:  1
Devices:
  ID        SIZE  PATH
   1     5.00GiB  /dev/sda2


root@ArchLabs / # mkfs.xfs -L home.xfs /dev/sda3
meta-data=/dev/sda3              isize=512    agcount=4, agsize=327680 blks
        =                       sectsz=512   attr=2, projid32bit=1
        =                       crc=1        finobt=1, sparse=0, rmapbt=0, reflink=0
data     =                       bsize=4096   blocks=1310720, imaxpct=25
        =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal log           bsize=4096   blocks=2560, version=2
        =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

root@ArchLabs / # mkswap -L swap.swap /dev/sda4
Setting up swapspace version 1, size = 2.5 GiB (2683281408 bytes)
LABEL=swap.swap, UUID=caffb094-e15e-4803-875f-b5a2b7152abc

5. Create btrfs subvolumes:
   1. mount /dev/sda2 /mnt
   2. btrfs subvolume create /mnt/@
   3. btrfs subvolume create /mnt/@pkg
   4. btrfs subvolume create /mnt/@log
   5. btrfs subvolume create /mnt/@snapshots
   6. umount /mnt

root@ArchLabs / # mount /dev/sda2 /mnt
root@ArchLabs / # btrfs subvolume create /mnt/@
Create subvolume '/mnt/@'
root@ArchLabs / # btrfs subvolume create /mnt/@pkg
Create subvolume '/mnt/@pkg'
root@ArchLabs / # btrfs subvolume create /mnt/@log
Create subvolume '/mnt/@log'
root@ArchLabs / # btrfs subvolume create /mnt/@snapshots
Create subvolume '/mnt/@snapshots'
root@ArchLabs / # ls /mnt
@  @log  @pkg  @snapshots
root@ArchLabs / # umount /mnt

6. Create folders and mount btrfs subvolumes:
   1. mount -o noatime,compress=lzo,space_cache,ssd,subvol=@ /dev/sda2 /mnt
   2. mkdir -p /mnt/{btrfs,boot,home,var/log,var/cache/pacman/pkg,.snapshots}
   3. mount -o noatime,compress=lzo,space_cache=v2,ssd,subvolid=5 /dev/sda2 /mnt/btrfs
   4. mount -o noatime,compress=lzo,space_cache=v2,ssd,subvol=@snapshots /dev/sda2 /mnt/.snapshots
   5. mount -o noatime,compress=lzo,space_cache=v2,ssd,subvol=@pkg /dev/sda2 /mnt/var/cache/pacman/pkg/
   6. mount -o noatime,compress=lzo,space_cache=v2,ssd,subvol=@log /dev/sda2 /mnt/var/log/

root@ArchLabs / # mount -o noatime,compress=lzo,space_cache,ssd,subvol=@ /dev/sda2 /mnt
root@ArchLabs / # mkdir -p /mnt/{btrfs,boot,home,var/log,var/cache/pacman/pkg,.snapshots}
root@ArchLabs / # ls /mnt
boot  btrfs  home  var

root@ArchLabs / # mount -o noatime,compress=lzo,space_cache=v2,ssd,subvolid=5 /dev/sda2 /mnt/btrfs
root@ArchLabs / # mount -o noatime,compress=lzo,space_cache=v2,ssd,subvol=@snapshots /dev/sda2 /mnt/.snapshots
root@ArchLabs / # mount -o noatime,compress=lzo,space_cache=v2,ssd,subvol=@pkg /dev/sda2 /mnt/var/cache/pacman/pkg/
root@ArchLabs / # mount -o noatime,compress=lzo,space_cache=v2,ssd,subvol=@log /dev/sda2 /mnt/var/log/

7. Mount boot, home and enable swap partition:
   1. mount /dev/sda1 /mnt/boot
   2. mount /dev/sda3 /mnt/home
   3. swapoff -a && swapon /dev/sda4

root@ArchLabs / # mount /dev/sda1 /mnt/boot
root@ArchLabs / # mount /dev/sda3 /mnt/home
root@ArchLabs / # swapoff -a && swapon /dev/sda4

8. Verify setup with:
   1. mount
   2. swapon -s

root@ArchLabs / # mount
...
/dev/sda2 on /mnt type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=257,subvol=/@)
/dev/sda2 on /mnt/btrfs type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=5,subvol=/)
/dev/sda2 on /mnt/.snapshots type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=260,subvol=/@snapshots)
/dev/sda2 on /mnt/var/cache/pacman/pkg type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=258,subvol=/@pkg)
/dev/sda2 on /mnt/var/log type btrfs (rw,noatime,compress=lzo,ssd,space_cache,subvolid=259,subvol=/@log)
/dev/sda1 on /mnt/boot type ext4 (rw,relatime,data=ordered)
/dev/sda3 on /mnt/home type xfs (rw,relatime,attr2,inode64,noquota)

root@ArchLabs / # swapon -s
Filename                                Type            Size    Used    Priority
/dev/sda4                               partition       2620392 0       -2

9. Leave terminal by pressing ALT+[KEY-LEFT] two times.
10. Go Back and Install ArchLabs.
11. Wait a few minutes.
12. Select as Bootloader Device your partition /dev/sda.
13. Set System Hostname, Locale + Timezone and Root Password.
14. Create a New User and apply System Tweaks (if required).
15. Go Back, click on Done and restart into your (new) ArchLabs system.

florian@myarchlabs ~ % df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
dev            devtmpfs  2.0G     0  2.0G   0% /dev
run            tmpfs     2.0G  476K  2.0G   1% /run
/dev/sda2      btrfs     5.0G  2.9G  1.8G  62% /
tmpfs          tmpfs     2.0G     0  2.0G   0% /dev/shm
tmpfs          tmpfs     2.0G     0  2.0G   0% /sys/fs/cgroup
tmpfs          tmpfs     2.0G  6.3M  2.0G   1% /tmp
/dev/sda2      btrfs     5.0G  2.9G  1.8G  62% /btrfs
/dev/sda2      btrfs     5.0G  2.9G  1.8G  62% /var/cache/pacman/pkg
/dev/sda2      btrfs     5.0G  2.9G  1.8G  62% /var/log
/dev/sda2      btrfs     5.0G  2.9G  1.8G  62% /.snapshots
/dev/sda1      ext4      488M   54M  400M  12% /boot
/dev/sda3      xfs       5.0G   50M  5.0G   1% /home
tmpfs          tmpfs     395M  8.0K  395M   1% /run/user/1000

florian@myarchlabs ~ % ls /btrfs
@  @log  @pkg  @snapshots

florian@myarchlabs ~ % cat /etc/fstab
# /dev/sda2 LABEL=root.btrfs
UUID=0fdbc292-a9b2-4862-a4be-61b019db243a       /               btrfs           rw,noatime,compress=lzo,ssd,space_cache,subvolid=257,subvol=/@,subvol=@      0 0

# /dev/sda2 LABEL=root.btrfs
UUID=0fdbc292-a9b2-4862-a4be-61b019db243a       /btrfs          btrfs           rw,noatime,compress=lzo,ssd,space_cache,subvolid=5,subvol=/  0 0

# /dev/sda2 LABEL=root.btrfs
UUID=0fdbc292-a9b2-4862-a4be-61b019db243a       /.snapshots     btrfs           rw,noatime,compress=lzo,ssd,space_cache,subvolid=260,subvol=/@snapshots,subvol=@snapshots    0 0

# /dev/sda2 LABEL=root.btrfs
UUID=0fdbc292-a9b2-4862-a4be-61b019db243a       /var/cache/pacman/pkg   btrfs           rw,noatime,compress=lzo,ssd,space_cache,subvolid=258,subvol=/@pkg,subvol=@pkg        0 0

# /dev/sda2 LABEL=root.btrfs
UUID=0fdbc292-a9b2-4862-a4be-61b019db243a       /var/log        btrfs           rw,noatime,compress=lzo,ssd,space_cache,subvolid=259,subvol=/@log,subvol=@log        0 0

# /dev/sda1 LABEL=boot.ext4
UUID=2de35468-684c-4804-a29c-51aa708bf5e0       /boot           ext4            rw,relatime,data=ordered        0 2

# /dev/sda3 LABEL=home.xfs
UUID=dae18c6c-4361-4e73-a775-102662f08ff5       /home           xfs             rw,relatime,attr2,inode64,noquota   0 2

# /dev/sda4 LABEL=swap.swap
UUID=caffb094-e15e-4803-875f-b5a2b7152abc       none            swap            defaults,pri=-2 0 0

Acknowledgements

1. Btrfs for this great and powerful filesystem,
2. unicks.eu aka Nick for all the great videos about GNU/Linux!

ToDoList

1. Fix Markdown Syntax.