LVM

Linux's Logical Volume Manager (LVM) provides means to dynamically organize partitions. For example, partitions can be created, moved, resized and deleted regardless of fragmentation on disk.

Basic Setup

LVM manages four basic building blocks:

Physical volume (PV)

Unix block device node reserved for use by LVM. Examples: a disk partition, a whole disk (i.e. without a partition table), a meta device, or a loopback file.

Volume group (VG)

Group of PVs for allocating PEs to LVs.

Logical volume (LV)

"Virtual partition" composed of PEs inside a VG.

Physical extent (PE)

The smallest contiguous extent (default 4 MiB) in the physical volume that can be allocated to an LV.

Create an LV

Below are shell commands to setup a PV, a VG and an LV from scratch. LVM comes enabled by default on NixOS.

# format the partion into a physical volume (check with pvdisplay)
pvcreate /dev/sda2
# create a new volume group named pool (check with vgdisplay)
vgcreate pool /dev/sda2
# create a new logical volume named "home" with the size of 10GB (check with lvdisplay)
# makes /dev/pool/home available
lvcreate --size 10G --name home pool
# finally, create a filesystem inside the logical volume
mkfs.ext4 /dev/pool/home

Use the LV

Mount the filesystem on the logical volume by adding an entry to the fileSystems NixOS option.

❄︎ /etc/nixos/configuration.nix

fileSystems."/home" = {
  device = "/dev/pool/home";
  fsType = "ext4";
};

Booting with special LVM Modes

LVM provides a number of special features such as creating snapshots, RAID for single LVs and much more. If you want to use these devices on bootup, the associated dm-* kernel module must be provided in the initrd (see for example Nixpkgs issue 🚩︎#33646). Here is a non-exhaustive list of features and the corresponding kernel module and other options to put into your NixOS configuration:

❄︎ /etc/nixos/configuration.nix

boot.initrd.kernelModules = [
  "dm-snapshot" # when you are using snapshots
  "dm-raid" # e.g. when you are configuring raid1 via: `lvconvert -m1 /dev/pool/home`
  "dm-cache-default" # when using volumes set up with lvmcache
];

services.lvm.boot.thin.enable = true; # when using thin provisioning or caching

Automated Partitioning

People have created a number of tools to automate the partitioning in NixOS:

NixOps

NixOps can repartition Hetzner Physical Machines, see NixOps Manual.

Disko

Disko provides means to automatically generate the creation and configuration of logical volumes, see https://github.com/nix-community/disko

Troubleshooting

LVM keeps ZFS zvol busy

The LVM service will possibly manage zfs zvol devices. If you want to destroy a zvol which contains a vm which uses LVM it can give an error.

zfs destroy -r vmstorage/vm-alma
dataset is busy

Issue lvs to see if the zvol device is in use.

The nixpkgs options of LVM (26.05) does not provide an entry where you could enter your own configurations in /etc/lvm/lvm.conf. Other nixpkgss options will use the lvm.conf also.

A workaround is to define a device filter which instructs lvm no to use zfs zvols:

devices {
filter="r|^/dev/zvol*|"
global_filter="r|^/dev/zvol*|"
}

Setup a lvm.conf in NixOS such as:

environment.etc."lvm/lvm.conf".text = pkgs.lib.mkForce ''
devices { 
    filter = [ "r|^/dev/zvol*|" ]
    global_filter = [ "r|^/dev/zvol*|" ]
}
'';

After reboot remove the zfs zvol. The workaround needs to be removed if NixOS previousely had a lvm.conf.

See also

• The corresponding ArchWiki page