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I thought I would take a few minutes and share some of my experiences with Linux KVM. In this first post, I’d like to share some things about live migration.
First, live migration works great … if you have the right version of KVM. Kvm-69, which is included with Ubuntu Hardy, is broken. Starting with kvm-70, live migration works again and works very well. You hardly even notice at all if you’re logged into the machine remotely while it migrates. Unfortunately, kvm-75 broke live migration again. Keep an eye on the changelog for a note that it’s fixed before trying kvm-75.
Setting your servers up for live migration is trivial. I was very surprised at how easy it was. All you need to do use share ssh keys between the user running your VM on each server. Then, from your running VM, use ctrl-alt-2 to switch to the qemu monitor then run migrate ssh://other-vm-host. In a few moments, the command will return and the machine will be migrated. But, there’s a chance here to scare yourself here. If you jump back to the console (ctrl-alt-1), you’ll still see your VM but it will seem frozen. It actually is … on the original VM. If you log into the console from the other VM host, your console should be working as expected. Go back to the original console and run quit to end the session for the now migrated VM.
If you don’t want to wait for migration to complete, you can use the -d flag ( migrate -d ssh://other-vm-host) to tell the qemu monitor not to wait. You can watch the migration process with the qemu monitor command info migration. You can even cancel the migration if you decide that you really didn’t want to send your VM to another box.
A couple of things of note. First, the docs recommend that you use the VNC console for machines that you plan on migrating. Second, I have seem some very poor behavior of the mouse pointer on Windows guests after migration. Basically, the guest’s mouse does not track properly with the VNC pointer. I haven’t yet tried setting the virtual console size manually to see if that makes any difference. If you try it, let me know how it works out for you.
Update (12 May 2009): ssh migration has been removed in newer versions of KVM as part of the merge with qemu. (I don’t know the exact version. If someone knows, please drop a comment below.) This removed the one really easy way of migrating VMs between hosts. You can still migrate over SSH bit it requires a lot more work. From the qemu monitor you do something like this: migrate “exec:ssh remote qemu-system-x86_64 [all vm options] -incoming ‘exec:cat’”
Update (24 June 2009): I have confirmed that the migrate command above works with kvm-85. I suggest that you create a script to print out your kvm options and then you can do something like this: monitor “exec:ssh remote `get-kvm-cmd vmname` -incoming ‘exec:cat’”
According to aligouri in #kvm on Freenode, nobody used migrate ssh: and I was the first person to complain about it’s removal. You may want to take a look at libvirt (which now supports live migration) if you want easy to use live migration.