Tuesday, October 23, 2012

Linux KVM Disk Drivers

I was having a problem with storage device names on virtual machines running on a RedHat KVM host.  Occasionally, I'd build a VM and the storage device would be named /dev/sda and other times /dev/xvda.  I quickly found that if I created the VM using virt-install, I got a xvda device, and if I used virt-manager (the GUI app), I got the sda device. After some investigation, I've discovered where things went wrong.

First, the syntax of the virt-install command changed in RHEL 6, and I was still using the RHEL 5 command.  Rather than complaining, RHEL 6 would guess what it thought I meant.  Here's the wrong command:
virt-install -n server -r 512 -w bridge=virbr0 \
-f /var/lib/libvirt/images/server.img -s 10
The -f/-s options says to create an image file the is 10GB.
Here's what was implemented:
virt-install -n server -r 512 -w bridge=virbr0 \
-disk path=/var/lib/libvirt/images/server.img,\
Rather than complaining that the -f/-s options were deprecated, it invoked the new syntax and assumed I wanted an IDE drive, which on RHEL 6, is named as if it were an SCSI device.  We can force the paravirt driver by using the correct command:
virt-install -n server -r 512 -w bridge=virbr0 \
-disk path=/var/lib/libvirt/images/server.img,\
Second, the GUI does not allow a VM's disk type to be selected from the install wizard-- it always defaults to the paravirt driver.  To force a specific driver, on the last screen of the wizard, check the box for "Customize configuration before install".

This will open a new window listing the VM's hardware.  Select "Add Hardware", select "storage", and configure a second disk the same size as the first.  At this point, there is a pull down menu that will specify the driver.  Once the second disk is in place, remove the first.  Removing the first disk before the adding the replacement disk can cause problems.

Hint: Once you've modified the hardware, there is not "Apply" option.  Just close the window and the VM will launch.

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