Linux, Technical

Live Migrate VM with Local Storage: virsh migrate

In my previous post I lamented that it seemed I could not migrate a VM between virtual hosts without shared storage. This is incorrect. I have just completed a test run based on an old mailing list message:

ServerA> virsh -c qemu:///system list
 Id    Name                           State
 5     testvm0                       running

ServerA> virsh -c qemu+ssh://ServerB list # Test connection to ServerB
 Id    Name                           State

The real trick is to work around a bug whereby the target disk needs to be created. So, on ServerB, I created an empty 8G disk image:

ServerB> sudo dd if=/dev/zero of=/var/lib/libvirt/images/testvm0.img bs=1 count=0 seek=8G

Or, if you’re into backticks and remote pipes:

ServerB> sudo dd if=/dev/zero of=/var/lib/libvirt/images/testvm0.img bs=1 count=0 seek=`ssh ServerA ls -l /var/lib/libvirt/images/testvm0.img | awk '{print $5}'`

Now, we can cook with propane:

ServerA> virsh -c qemu:///system migrate --live --copy-storage-all --persistent --undefinesource testvm0 qemu+ssh://ServerB/system

This will take some time. I used virt-manager to attach to testvm0’s console and ran a ping test. At the end of the procedure, virt-manager lost console. I reconnected via ServerB and found that the VM hadn’t missed a ping.

And now:

ServerA> virsh -c qemu+ssh://ServerB/system list
 Id    Name                           State
 4     testvm0                        running

Admittedly, the need to manually create the target disk is a little more janky than I’d like, but this is definitely a nice proof of concept and another nail in the coffin of NAS being a hard dependency for VM infrastructure. Any day you can kiss expensive, proprietary SPOF dependencies goodbye is a good day.

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Categories: Linux, Technical

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