How do I extend the disk space for a KVM guest?

Each guest is running logical volume manager. If you need to allocate more disk space on a guest, run the vgs command to determine if you already have un-allocated disk space available.

This example shows that chavez has space available:

0 chavez:~# vgs
  VG         #PV #LV #SN Attr   VSize   VFree 
  vg_chavez0   1   7   0 wz--n- 499.52g 57.52g
0 chavez:~#

There's no need to allocate more space from the host, since chavez already has 57GB free.

Here's an example of a host without any available disk space:

0 mandela:~# vgs
  VG          #PV #LV #SN Attr   VSize  VFree
  vg_mandela0   1   6   0 wz--n- 69.52g    0 
0 mandela:~#

mandela has no available disk space, so additional space must be allocated from the host, ken.

Our KVM guests are each allocated a single logical volume from the KVM host's volume group (typically named after the host).

The first and easiest step is to login to the host, check available disk space:

0 ken:~# vgs
  VG      #PV #LV #SN Attr   VSize VFree
  vg_ken0   1  16   0 wz--n- 1.82t 1.29t
0 ken:~# 

ken has 1.29 TB available.

Next check how much space is already allocated to your guest:

0 ken:~# lvs | grep mandela 
  mandela  vg_ken0 -wi-ao  20.00g                                      
0 ken:~#

The, add more space:

lvextend --size 35G vg_ken0/mandela
  Extending logical volume mandela to 35.00 GiB
  Logical volume mandela successfully resized
0 ken:~#

Next, you will need to reboot the guest before the change in disk space is recognized by the guest (​this is apparently a requirement until someone implements online disk resizing). You will need to execute: shutdown -h now, so that the underlying block devices get reinitialized by a new process.

After rebooting, you can confirm the new size on the guest by running:

cat /proc/partitions

You should see that the base disk (e.g. /dev/vda) reflects the new size, although all of the partitions reflect their old sizes (e.g. /dev/vda2).

Next, on the guest, run parted and print the current partition table. It's often easier to manage if you look at units in sectors:

parted /dev/vda unit s p

Note carefully the start sector for the last partition. You will need this exact number in the step below.

Delete the last partition, and re-create it with a higher end sector (yes, you can do this without destroying the data on the disk). Replace STARTSECTOR with the start sector of the output from the command above (including the s after the number). Specifying -1 as the end sector means go as far to the end as possible. If you get a message saying that the sector you requested is not possible, accept a different one, you can safely say yes.

parted /dev/vda

The following commands are executed in interactive mode:

unit s
rm 2
mkpart primary STARTSECTOR -1 

Next, check to see if your changes are reflected in /proc/partitions:

cat /proc/partitions

You may need to reboot a second time before they will show up.

Once your partition table is reflecting the new size, run the following command:

pvresize /dev/vda2

And now your volume group should show additional free space.