1,798
edits
Changes
no edit summary
In this chapter of [[Xen Virtualization Essentials]] we will be looking at creating a root filesystem for a Xen domainU system on an NFS mounted filesystem. In many ways this is little different from other techniques covered in preceding chapters in that , although similar techniques can be used to populate the root filesystem. The difference comes at the point that the domainU system is booted. Instead of running on a local filesystem, the root filesystem for our domainU system is instead located on a remote server. This is particularly useful when performing live migrations of running Xen based systems from one machine to another.
== Kernel Requirements for NFS based Root Filesystems ==
There is one key requirement for booting a Xen domU with an NFS mounted root filesystem. By default, most Linux kernels are not configured to support NFS mounted root filesystems. It is essential, therefore, that a custom kernel with CONFIG_ROOT_NFS support enabled be built and referenced in the ''kernel='' field of the Xen config file for the guest domain.The mechanism for achieving this varies between Linux distributions. A number of distribution specific tutorials are available on the internet, once such tutorial being [http://www.virtuatopia.com/index.php/CentOS_5_-_Build_a_Custom_Xen_Kernel_with_NFS_Root_Filesystem_(CONFIG_ROOT_NFS)_Support CentOS 5 - Build a Custom Xen Kernel with NFS Root Filesystem (CONFIG ROOT NFS) Support]
== Populating the Root Filesystem ==
Once a custom Xen kernel with NFS root filesystem support enabled is available , the next step is to populate a suitable disk partition with a root filesystem. In this tutorial we will assume that this partition is mounted at /xen. A number of options for creating the root fielsystem are available and have been covered in preceding chapters. Recommended chapters to read for this are as follows:
* [[Building a Xen Virtual Guest Filesystem on a Physical Disk Partition (Cloning Host System)]]
</pre>
The ''rw'' indicates that the system with the specified IP address is able to read and write to the NFS mounted filesystem when it is mounted. The ''no_root_squash'' directive is used to allow the root user of the Xen domU to write to the NFS mounted root filesystem, (if this is not set they the root user will not have access to the guest system).
Once the ''/etc/exports'' file has been updated the ''exportfs'' command can be run to update the table of exported NFS filesystems. This command needs to be run as ''root'' using the '''-a''' flag:
Once we are on the remote system we need to copy the custom kernel with root NFS support enabled to the local system so that it can be referenced in our Xen configuration file. Let's assume the kernel is named vmlinuz-2.6.18-53.1.14.el5.myxen and we have copied it to /boot on the local system.
For the purposes of this tutorial we will also assume that the NFS server hosting our domainU root filesystem has an IP address of 192.168.2.12. You will also recall that the the root filesystem is mounted as /xen on our NFS server. With this information we can create our Xen configuration file which we will name xen_nfs.cfg:
<pre>
