Spherical Chicken

Three things need to happen to increase the disk available to a Linux VM:

1. Increase the disk size available to a virt in XenCenter
2. Increase the size of the VM partition in the VM’s partition table
3. Grow the size of the ext3 filesystem in the VM

First of all the VM needs to be shut down in order to extend the size of the drive in XenCenter (I don’t know how to do this on-the-fly).

On the storage tab of the VM, click on properties and increase the size of the storage in XenCenter

Increase the size of the linux / partition using fdisk. The utility parted should allow you to resize partitions as well and be much smarter about it and let you move filesystem data around as well, but parted does not allow you to edit running partitions. In this case since I want to edit /dev/xvda3 which is my / partition and it is at the end of the virtual disk I only need to extend the size of the partition and not move it around. On the running VM use fdisk to increase the partition:

# fdisk /dev/xvda

The number of cylinders for this disk is set to 5221.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): p

Disk /dev/xvda: 42.9 GB, 42949672960 bytes
255 heads, 63 sectors/track, 5221 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *           1          38      305203+  83  Linux
/dev/xvda2              39         103      522112+  82  Linux swap / Solaris
/dev/xvda3             104        4177    32724405   83  Linux

Command (m for help): d
Partition number (1-4): 3

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 3
First cylinder (104-5221, default 104): 104
Last cylinder or +size or +sizeM or +sizeK (104-5221, default 5221): 5221

Command (m for help): p

Disk /dev/xvda: 42.9 GB, 42949672960 bytes
255 heads, 63 sectors/track, 5221 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *           1          38      305203+  83  Linux
/dev/xvda2              39         103      522112+  82  Linux swap / Solaris
/dev/xvda3             104        5221    41110335   83  Linux

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table.
The new table will be used at the next reboot.
Syncing disks.
#

Now the last warning is accurate, you need to reboot the VM again in order for the kernel to pickup the changes to the partition geometry. Once the VM has rebooted, you can run resize2fs in the running VM which will automatically notice the difference between the partition geometry and the size of the filesystem and will extend the filesystem to its maximum size:

# resize2fs /dev/xvda3
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/xvda3 is mounted on /; on-line resizing required
Performing an on-line resize of /dev/xvda3 to 10277583 (4k) blocks.
The filesystem on /dev/xvda3 is now 10277583 blocks long.

The need to reboot the VM twice here is a little bit annoying, there should be a better way to do this on-the-fly. It should also be straightforwards to replace the use of the GUI with xe commands, and if you’re using ext3 volumes in XenServer and have access to the VHD files, it should be possible to do the partitioning and filesystem resizing from the dom0 as well (although doing this on-the-fly would require some kind of co-operation with the VM kernel, and probably requires the VM to be shut down anyway).