Smoothwall Firewall project

Monday, 5 December 2011

How to add another hard disk to your Xen virtual machine

If like me you have many Xen virtual machines that have been running for a while, you will likely come across the problem of having to increase the disk space, as the services or jobs the virtual machine offers over time changes.

Now you could increase the size of the existing disk, but I always feel that offers a level of risk, which with adding a new disk and using LVM or a new mount goes away.

Ok, so firstly we need to create a new virtual disk on the physical harddisk with the following command:

dd if=/dev/zero of=vm01-disk2.img bs=1G count=50

This will give us 50GB of new space. You can of course change the size to what ever you require, if your physical disk space will allow it.

Then we need to edit the Xen virtual machine configuration file and add in the new disk.

On my virtual machine host this is in /etc/xen/vm01

The line that needs to be altered is obviously the disk line, like so

disk = [ "tap:aio:/opt/xendisks/vm01.img,xvda,w",\
"tap:aio:/opt/xendisks/vm01-disk2.img,xvdb,w" ]

NB Notice the slash which allows the configuration line to expand over two lines in the configuration file.

Once you have done this, you can use the virsh and xm commands to restart the virtual machine , so that the new disk is available inside the virtual machine.

Once your VM has re-booted, you will notice another disk, which you can then either add to your LVM configuration, or just format and mount in a new directory. Here is my new mount as an example.

/dev/xvdb 50G 19G 29G 39% /opt/tomcat

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