LUKS and BTRFS make for an interesting combination if you ever need to perform some work on another drive. This is a step-by-step procedure I wrote for myself after needing it more than once…

The ArchWiki already covers how to do this if you combine a few separate articles, but I wanted a unified document.

Mounting

  1. Identify the drive to mount. 
     $ sudo lsblk
 NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
 ...output trimmed...
 nvme1n1     259:0    0 931.5G  0 disk 
 ├─nvme1n1p1 259:1    0     1G  0 part 
 └─nvme1n1p2 259:2    0 930.5G  0 part

 $ export DRIVE="nvme1n1"

    I have trimmed the output to show only the drive applicable here. I am also exporting it as a variable to use later.

  2. Decrypt the drive if encrypted. Otherwise, skip this step. 
     $ sudo --preserve-env=DRIVE cryptsetup luksOpen /dev/"${DRIVE}p2" luksvol
 Enter passphrase for /dev/nvme1n1p2:

    Note: If the prompt appears to freeze, be patient. It takes a few seconds to decrypt.

  3. Mount the @ subvolume. 
     $ sudo mount --options subvol=@ /dev/mapper/luksvol /mnt

    The @ subvolume is the / directory. It needs to be mounted first.

    NOTE: You may or may not have additional subvolumes, but it is safe to run the following commands regardless.

    The following message may appear if a subvolume doesn’t exist.

     mount: /mnt/.snapshots: fsconfig() failed: No such file or directory.
    1. Mount the @home subvolume. 
       $ sudo mount --options subvol=@home /dev/mapper/luksvol /mnt/home
    2. Mount the @pkg subvolume. 
       $ sudo mount --options subvol=@pkg /dev/mapper/luksvol /mnt/var/cache/pacman/pkg
    3. Mount the @log subvolume. 
       $ sudo mount --options subvol=@log /dev/mapper/luksvol /mnt/var/log
    4. Mount the @snapshots subvolume. 
       $ sudo mount --options subvol=@snapshots /dev/mapper/luksvol /mnt/.snapshots
  4. Mount the boot partition. This is critical if you want to update the kernel or bootloader. 
     $ sudo --preserve-env=DRIVE mount /dev/"${DRIVE}p1" /mnt/boot
  5. Finally, you can arch-chroot into the mounted system. 
     $ sudo arch-chroot /mnt

Cleanup

After exiting the arch-chroot, unmount everything.

$ sudo umount --recursive /mnt

If the drive was encrypted, close the LUKS volume.

$ sudo cryptsetup close luksvol

References:

  1. https://wiki.archlinux.org/title/Dm-crypt
  2. https://wiki.archlinux.org/title/Chroot#Running_on_Btrfs