The Book of Xen by Luke Crawford; Chris Takemura

Author:Luke Crawford; Chris Takemura
Language: eng
Format: mobi
Tags: Networking, Virtual computer systems, Reference:Computers, Electronic books, Computers, General, Parallel processing (Electronic computers), Virtualization, Xen (Electronic resource), Computer organization, Operating Systems
ISBN: 9781593271862
Publisher: No Starch Press
Published: 2009-04-28T21:29:37.353000+00:00

Chapter 9. XEN MIGRATION

In these situations the combination of virtualization and migration significantly improves manageability.

—Clark et al., "Live Migration of Virtual Machines"

So let's review: Xen, in poetic terms, is an abstraction, built atop other abstractions, wrapped around still further abstractions. The goal of all this abstraction is to ensure that you, in your snug and secure domU, never even have to think about the messy, noisy, fallible hardware that actually sends electrical pulses out the network ports.

Of course, once in a while the hardware becomes, for reasons of its own, unable to run Xen. Perhaps it's overloaded, or maybe it needs some preventive maintenance. So long as you have advance warning, even this need not interrupt your virtual machine. One benefit of the sort of total hardware independence offered by Xen is the ability to move an entire virtual machine instance to another machine and transparently resume operation—a process referred to as migration.

Xen migration transfers the entire virtual machine—the in-memory state of the kernel, all processes, and all application states. From the user's perspective, a live migration isn't even noticeable—at most, a few packets are dropped. This has the potential to make scheduled downtime a thing of the past. (Unscheduled downtime, like death and taxes, shows every sign of being inescapable.[51])

Migration may be either live or cold,[52] with the distinction based on whether the instance is running at the time of migration. In a live migration, the domain continues to run during transfer, and downtime is kept to a minimum. In a cold migration, the virtual machine is paused, saved, and sent to another physical machine.

In either of these cases, the saved machine will expect its IP address and ARP cache to work on the new subnet. This is no surprise, considering that the in-memory state of the network stack persists unchanged. Attempts to initiate live migration between different layer 2 subnets will fail outright. Cold migration between different subnets will work, in that the VM will successfully transfer but will most likely need to have its networking reconfigured. We'll mention these characteristics again later in our discussion of live migration.

First, though, let's examine a basic, manual method for moving a domain from one host to another.

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