Extending VM disk

Consider the following situation: a CentOS 6.2 host (this can also be Fedora or RHEL for that matter) using KVM to run virtual machines (VM). The host uses the Logical Volume Manager (LVM) as storage to create disks for the VMs. Now one of the machines needs more disk space then was originally envisioned and thus the space must be extended. The guest system in turn is also running CentOS 6.2 and itself using LVM to setup its storage.

So we have the following entities to deal with:
Host: The physical machine running CentOS 6.2 and KVM.
Guest: Virtual machine running in KVM.
vg_host/lv_vm: The logical volume on the host system which constitutes the disk drive of the Guest.
/dev/vda2: The second partition on the first virtio disk in the Guest. It contains the only physical volume used on the Guest.
vg_vm: The volume group on the Guest where the only physical volume is /dev/vda2.
vg_vm/lv_root: The logical volume in the Guest volume group which facilitates the root of Guest's filesystem.

To keep things simple we assume that there is only a single mount point for the Guest, i.e. /usr etc. are not split off onto own partitions or logical volumes. Replace vg_host, lv_vm, and vg_vm with the actual values from your system. vgdisplay and lvdisplay can help you to find out about it.

So the following steps are necessary to get more disk space into the VM.

1. On Host: Extend vg_host/lv_vm
2. On Guest Extend /dev/vda2 partition
3. On Guest Extend /dev/vda2 physical volume
4. On Guest Extend vg_vm/lv_root logical volume
5. On Guest Extend vg_vm/lv_root file system

Therefore, execute the following steps.

0. Stop the VM:

virsh shutdown VM

1. On Host: Extend vg_host/lv_vm: this code extends by 10GB, change as appropriate, make sure there is enough free space left in the volume group.

lvresize -L+10G vg_host/lv_vm

2. On Guest Extend /dev/vda2 partition: This is critical, take special care! It is important to choose the proper starting cylinder. The suggested start might be wrong, therefore first print the current start and keep it. Sorry this is the German output, but you should be able to match it properly.

fdisk /dev/vda2
Befehl (m für Hilfe): p
   Gerät  boot.     Anfang        Ende     Blöcke   Id  System
/dev/vda1   *           3        1018      512000   83  Linux
Partition 1 endet nicht an einer Zylindergrenze.
/dev/vda2            1018       21391    10267648   8e  Linux LVM
Partition 2 endet nicht an einer Zylindergrenze.
Befehl (m für Hilfe): d
Partitionsnummer (1-4): 2

Befehl (m für Hilfe): n
Befehl  Aktion
   e      Erweiterte
   p      Primäre Partition (1-4)
p
Partitionsnummer (1-4): 2
Erster Zylinder (1-62415, Vorgabe: 1): 1018
Last Zylinder, +Zylinder or +size{K,M,G} (1018-62415, Vorgabe: 62415): 
Benutze den Standardwert 62415

Befehl (m für Hilfe): p

Platte /dev/vda: 32.2 GByte, 32212254720 Byte
16 Köpfe, 63 Sektoren/Spur, 62415 Zylinder
Einheiten = Zylinder von 1008 × 512 = 516096 Bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000407f8

   Gerät  boot.     Anfang        Ende     Blöcke   Id  System
/dev/vda1   *           3        1018      512000   83  Linux
Partition 1 endet nicht an einer Zylindergrenze.
/dev/vda2            1018       62415    30944136   83  Linux

You now need to reboot because the partition is currently in use.

3. On Guest Extend /dev/vda2 physical volume: extend to maximal size. Make sure you have rebooted your system or otherwise this won't work (even though it doesn't report any errors).

# pvresize /dev/vda2
  Physical volume "/dev/vda2" changed
  1 physical volume(s) resized / 0 physical volume(s) not resized

4. On Guest Extend vg_vm/lv_root logical volume

lvresize -l+100%FREE vg_vm/lv_root

5. On Guest Extend vg_vm/lv_root file system: this can be done while the file system is mounted and active. But you'd probably be screwed on a power failure or so.

resize2fs vg_vm/lv_root

Done! Now df -h should report the full space.