Shrink root LVM on LUKS2 btrfs root partition to enlarge swap partition

Question

Here's my setup

$ lsblk
NAME                   MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
nvme0n1                259:0    0 238.5G  0 disk
├─nvme0n1p1            259:1    0   100M  0 part  /boot/efi
├─nvme0n1p2            259:2    0   250M  0 part  /boot
└─nvme0n1p3            259:3    0 238.1G  0 part
  └─Be-Water-My-Friend 254:0    0 238.1G  0 crypt
    ├─Arch-swap        254:1    0     2G  0 lvm   [SWAP]
    └─Arch-root        254:2    0 236.1G  0 lvm   /

I have one main LUKS2-encrypted partition (nvme0n1p3), with one LVM volume group (Be-Water-My-Friend) containing two logical volumes Arch-swap and Arch-root.

The Arch-root is a btrfs.

When I set that up, I only chose 2GB of swap which turns out to be insufficient for my needs. I would like to increase that to 24GB of swap.

For that, I think I need to

1. boot on a USB live key
2. decrypt the LUKS2 partition
3. mount the Arch-root volume
4. shrink the Arch-root file system with

   btrfs filesystem resize -22g
   

5. remove the Arch-swap logical volume
6. recreate the Arch-swap logical volume taking all available space in the Be-Water-My-Friend volume group.

Is there anything I'm missing? I really don't want to screw that up!

Answer 1

You need one extra step between 4 and 5 -- shrink the Arch-root logical volume using lvresize -L-22G Arch/root (lvresize has option --resizefs to resize both the LV and the filesystem, but it currently doesn't support btrfs so you can't use it here). This answer nicely explains difference between resizing filesystem (btrfs in your case) and the block device (LVM logical volume).

You also might want to use --uuid with mkswap to set your old swap UUID for the new swap. Swaps are usually not referred with UUID in /etc/fstab and GRUB, but using the old UUID might save you some problems.

Also if you just want a bigger swap, you can create a swap file on btrfs and use it as second swap.

Answer 2

A small correction:

Step 2 should be: unlock the LUKS2 partition (with a cryptsetup open --type luks ... or equivalent command).

Step 2.5 should be added: verify the presence of the Arch volume group and ensure it's been activated, with e.g. vgscan; vgchange -ay Arch. (Depending on which live media you're using, the system may or may not auto-activate the VG as the encrypted partition is unlocked.)

Step 4.5 should be added: shrink the LV after you're sure the filesystem inside it has been successfully shrunk:

lvresize -L-22G Arch/root

---

Also, Be-Water-My-Friend is the name throuch which the encrypted device is accessed, and thus also the name of the LVM Physical Volume (/dev/mapper/Be-Water-My-Friend).

The name of the LVM volume group is simply Arch. In lsblk listing, the names of LVM Logical Volumes are presented in the form VGname-LVname, so e.g. Arch-swap means logical volume swap in volume group Arch.

If the VG/LV name includes hyphens (minus signs), those will be doubled to distinguish from the single hyphen that acts as a delimiter between the names of the VG and the LV.