Using LVM for an entire hard drive

June 24, 2021 Tech Hardware lvcreate lvdisplay lvm pvcreate ubuntu ubuntu server vgcreate vgdisplay

On my new KVM system, I had a SSD for the Ubuntu OS, but wanted a separate LVM Logical Volume for the KVM-related files/folder, such as ISO files, KVM images, etc.

LVM understands hard drives without a partition table, and you can use the entire drive for LVM without partitioning. The drawback is that we'll have to use the whole device as physical volume, but that's exactly what I want to achieve.

Steps

Erase the current partition table on the drive, which isn't necessary with newer versions of pvcreate, but doesn't hurt anything:

sudo dd if=/dev/zero of=/dev/sdb bs=512 count=1
1+0 records in
1+0 records out
512 bytes copied, 0.000714602 s, 716 kB/s

Create the Physical Volume (PV):

sudo pvcreate /dev/sdb
Physical volume "/dev/sdb" successfully created.

Create the Volume Group:

sudo vgcreate kvm-vg /dev/sdb
  Volume group "kvm-vg" successfully created

Display the new Volume Group (VG):

sudo vgdisplay /dev/kvm-vg
  --- Volume group ---
  VG Name               kvm-vg
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  1
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                0
  Open LV               0
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               931.51 GiB
  PE Size               4.00 MiB
  Total PE              238467
  Alloc PE / Size       0 / 0
  Free  PE / Size       238467 / 931.51 GiB
  VG UUID               4F1YAS-cprX-inKs-eMdp-5aYy-tfCx-e6oAmB

Create the Logical Volume (LV), using all of the space on the VG (drive):
```
sudo lvcreate -n kvm-lv -l 100%FREE kvm-vg
```

Display the new Logical Volume we just created:

sudo lvdisplay kvm-vg/kvm-lv
  --- Logical volume ---
  LV Path                /dev/kvm-vg/kvm-lv
  LV Name                kvm-lv
  VG Name                kvm-vg
  LV UUID                Nu21c6-Bqhm-9eyN-07EU-o1UH-0UV3-kDkplg
  LV Write Access        read/write
  LV Creation host, time ubuntu-kvm, 2021-06-09 20:34:21 +0000
  LV Status              available
  # open                 0
  LV Size                931.51 GiB
  Current LE             238467
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

Format the Logical Volume:

sudo mkfs.ext4 /dev/kvm-vg/kvm-lv
mke2fs 1.45.5 (07-Jan-2020)
Creating filesystem with 244190208 4k blocks and 61054976 inodes
Filesystem UUID: 6ac3e69e-26bf-41c4-80f8-1cf255617584
Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
    4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
    102400000, 214990848

Allocating group tables: done
Writing inode tables: done
Creating journal (262144 blocks): done
Writing superblocks and filesystem accounting information: done

Create the '/kvm' directory, where we will keep all of our KVM-related files (ISOs, VMs, etc):
```
sudo mkdir /kvm
```

Get the block ID of the newly created LV:

sudo blkid
/dev/sda2: UUID="2e0048b6-113d-46b3-9ee5-7dc82f55647e" TYPE="ext4" PARTUUID="a5c6d238-ab41-40a2-b6ac-7094d8f29353"
/dev/sda3: UUID="xiRfps-E3kw-YvcU-zsWN-FHG2-kIhC-I03vEv" TYPE="LVM2_member" PARTUUID="c5a52c6f-4bf7-49c8-b3eb-5a4c9ec96320"
/dev/mapper/ubuntu--vg-ubuntu--lv: UUID="6026a797-54ab-49f0-a224-471298225cf5" TYPE="ext4"
/dev/loop0: TYPE="squashfs"
/dev/loop1: TYPE="squashfs"
/dev/loop2: TYPE="squashfs"
/dev/sdb: UUID="J4nd3U-fbRo-4CrX-a24z-Vcoh-X6Zf-C7iIae" TYPE="LVM2_member"
/dev/sda1: PARTUUID="84c8ca8d-0f2a-4700-b168-676b8e34d18a"
/dev/mapper/kvm--vg-kvm--lv: UUID="6ac3e69e-26bf-41c4-80f8-1cf255617584" TYPE="ext4"

The LV we just created has a UUID of 6ac3e69e-26bf-41c4-80f8-1cf255617584

Verify that the UUID exists in the /dev/disk/by-uuid/ directory:

ls -l /dev/disk/by-uuid/6ac3e69e-26bf-41c4-80f8-1cf255617584
lrwxrwxrwx 1 root root 10 Jun  9 20:53 /dev/disk/by-uuid/6ac3e69e-26bf-41c4-80f8-1cf255617584 -> ../../dm-1

Create a fstab entry for the LV, so it gets mounted as /kvm on bootup, by adding this line to the bottom of the /etc/fstab file.

We can do this by adding this line to /etc/fstab as root (must be root and not using sudo):
```
/dev/disk/by-uuid/6ac3e69e-26bf-41c4-80f8-1cf255617584 /kvm ext4 defaults 0 0
```
Reload fstab, so the new mount point shows up:
```
sudo mount -a
```

Verify the new directory shows up:

df -h /kvm
Filesystem                   Size  Used Avail Use% Mounted on
/dev/mapper/kvm--vg-kvm--lv  916G   77M  870G   1% /kvm

Not necessary, but I always like to double check, reboot, then verify that /kvm is mounted after rebooting.

References:

Man page for pvcreate http://manpages.ubuntu.com/manpages/focal/en/man8/pvcreate.8.html

LVM set up on Ubuntu https://www.maketecheasier.com/lvm-set-up-ubuntu/