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<table border="0" cellspacing="0" width="100%"><tr><td width="20%">[[Running and Connecting to VNC Servers on a Xen Guest (domainU) System|Previous]]<td align="center">[[Xen Virtualization Essentials|Table of Contents]]<td width="20%" align="right">[[Xen Monitoring Tools and Techniques|Next]]</td><tr><td width="20%">Running and Connecting to VNC Servers on a Xen Guest (domainU) System<td align="center"><td width="20%" align="right">Xen Monitoring Tools and Techniques</td></table><hr> <htmlet>xen</htmlet> Most Xen domainU guests are configured with access to a number of devices, typically for the root filesystem and swap space. it It is not uncommon, however, to need to provide an executing domainU system with temporary access to block devices such as disks and CD/DVD drives.
Fortunately Xen provides considerable flexibility in terms of adding devices to a running domainU guest system and subsequently detaching them when they are no longer required. This chapter will cover the basics of this topic area.
* ''<Mode>'' is the read/write mode under which the device is to be attached. Options are ''r'' for read-only, ''w'' for read/write and ''w!'' for read/write with sharing.
== Attaching a Device to a domainU Guest ==
With the above information in mind we can now attach a device to a domainU guest. The first piece of information we need is the ID of the guest domain. This can be obtained using the ''xm list'' command. The following output shows only one guest running on our system and it has an ID of 3:
<pre>
xm list
Name ID Mem VCPUs State Time(s)
Domain-0 0 875 1 r----- 790.2
XenGuest1 3 128 1 ------ 190.5
</pre>
Next we need to know the name by which the device to be added is accessed on the domain0 host (otherwise known as the ''backend device''). For the purposes of this chapter we will assume that we are adding a DVD drive that is visible to the domain0 host system as ''/dev/sr0''. This could equally be a disk drive partition such as ''/dev/hdb1'' or ''/dev/sda3''.
Before performing the attachment, we need to decide on a device name by which the device will be accessible on the domainU system (known as the ''frontend device''). As mentioned previously the device name must be an ''xvd'' device (for example ''/dev/xvda2''). Failure to use an ''xvd'' device type will result in the device not appearing in the domainU guest. For this chapter we will specify ''/dev/xvda2''.
With the above information gathered we will attach the device. Since we are attaching a DVD drive we will specify read-only mode:
<pre>
xm block-attach 3 phy:/dev/sr0 /dev/xvda2 r
</pre>
The device is now attached the domainU system with an ID of 3 and should be accessible from within that guest environment as ''/dev/xvda2''.
== Mounting the Device in the domainU Guest ==
Once the device attachment has been performed the next step is to mount the device on the domainU system. Log into the domainU guest and check the new device is visible and configured:
<pre>
ls -l /dev/xvda2
brw-r----- 1 root disk 202, 2 2008-05-02 15:46 /dev/xvda2
</pre>
With the device attached it is ready to be mounted:
<pre>
mkdir /tmp/mount
mount /dev/xvda2 /tmp/mnt
</pre>
== Detaching a Device from the domainU Guest ==
To detach a block device from the domainU first unmount it from the guest:
<pre>
umount /tmp/mnt
</pre>
Once unmounted from the domainU system move to the domain0 host and detach it using the ''xm block-detach'' specifying the domainU ID and the ''frontend'' device name (in this case ''/dev/xvda2''):
<pre>
xm block-detach 3 /dev/xvda2
</pre>
<htmlet>xen</htmlet>
<hr>
<table border="0" cellspacing="0" width="100%">
<tr>
<td width="20%">[[Running and Connecting to VNC Servers on a Xen Guest (domainU) System|Previous]]<td align="center">[[Xen Virtualization Essentials|Table of Contents]]<td width="20%" align="right">[[Xen Monitoring Tools and Techniques|Next]]</td>
<tr>
<td width="20%">Running and Connecting to VNC Servers on a Xen Guest (domainU) System<td align="center"><td width="20%" align="right">Xen Monitoring Tools and Techniques</td>
</table>
