Mount the filesystems

Now assemble the subvolumes into the directory tree that the new system will see, rooted at /mnt.

Pick the mount options once:

OPTS="noatime,compress=zstd:1"

noatime stops the filesystem from writing an access timestamp every time a file is read, which is a pure write amplification with almost no upside on a workstation. compress=zstd:1 compresses data transparently; level 1 is deliberate, because btrfs compression is fast enough at low levels to be effectively free, while higher levels cost noticeably more CPU for a few percent more space.

Mount everything

mount -o "$OPTS,subvol=@" "$ROOTDEV" /mnt
mkdir -p /mnt/{home,.snapshots,var/log,var/cache/pacman/pkg,swap,boot}
mount -o "$OPTS,subvol=@home"      "$ROOTDEV" /mnt/home
mount -o "$OPTS,subvol=@snapshots" "$ROOTDEV" /mnt/.snapshots
mount -o "$OPTS,subvol=@log"       "$ROOTDEV" /mnt/var/log
mount -o "$OPTS,subvol=@pkg"       "$ROOTDEV" /mnt/var/cache/pacman/pkg
mount -o "noatime,subvol=@swap"    "$ROOTDEV" /mnt/swap
mount "$ESP" /mnt/boot

Order matters: @ has to be mounted first, because the mkdir that follows creates the mount points inside it, and each later mount lands on one of those directories.

@swap is mounted without compress, deliberately. A swapfile has to be a plain, uncompressed, un-copy-on-write extent, and giving the subvolume compressed mount options would conflict with that.

Check it

findmnt -nR /mnt -o TARGET,SOURCE,FSTYPE

/mnt                        /dev/mapper/cryptroot[/@]          btrfs
├─/mnt/home                 /dev/mapper/cryptroot[/@home]      btrfs
├─/mnt/.snapshots           /dev/mapper/cryptroot[/@snapshots] btrfs
├─/mnt/var/log              /dev/mapper/cryptroot[/@log]       btrfs
├─/mnt/var/cache/pacman/pkg /dev/mapper/cryptroot[/@pkg]       btrfs
├─/mnt/swap                 /dev/mapper/cryptroot[/@swap]      btrfs
└─/mnt/boot                 /dev/nvme0n1p1                     vfat

Seven lines, with the subvolume in brackets after each source. If one is missing, or two targets show the same subvolume, fix it now – this tree is exactly what gets written into /etc/fstab in the next chapter, and a mistake here becomes a system that does not boot.