Server virtualization software is hot. But "hot" migration of virtual machines may make virtualization even, well, hotter.

Hot (a.k.a. live) migration refers to the ability to move a running virtual machine from one physical server to another. VMware plunged into live migration first, three years ago, with its combination of VMotion, its virtual machine transfer agent, and its Virtual Center management software. Because VMware supplies its Virtual Machine File System as an overlay for the storage system to allow the transfer of virtual machines, VMotion only works where two servers are accessing the same VMFS files.

XenSource, supplier of the open source Xen hypervisor, is launching its live migration feature, XenMotion, next month in XenEnterprise 4.1, and it will incorporate storage management capabilities from Symantec's Veritas software. Gordon Mangione, a XenSource senior VP, says that's a coup because many Xen users are already users of the Veritas File System, which will avoid forcing customers to adopt a new file management system.

Microsoft plans to include live migration in its Veridian virtualization hypervisor but can't yet refer to a specific availability date, says Patrick O'Rourke, manager in the Windows server division.

A GODLIKE FEATURE

VMotion and Virtual Center let Harold Shapiro move virtual machines from one physical server to another in his Warner Brothers Entertainment data center, where he's the technology architect. He calls it "a godlike feature" of virtualization.

The process takes a few hundredths of a second, because only a limited amount of data makes the transfer. The underlying virtual machine files aren't transferred; instead, the two physical servers involved are attached to the same disk storage pool. The new VM host can access needed files at the point where the other left off.

Originally intended as failover technology, live migration has many advantages. IT managers like the flexibility to be able to take down a working server during the day, rather than at night or on weekends, upgrade the operating system, apply patches, etc., then bring it back up again during regular working hours. In "green" data centers, operations managers look at where they have heavy workloads and move virtual machines around so that the cooling system isn't working excessively hard trying to keep just part of the data center at the right temperature.

Unfortunately, there are limitations, and significant ones. For example, before VMotion can make a transfer, Virtual Center checks that the x86 instruction set on the destination server is compatible with the instruction set on the origin server. Advanced Micro Devices and Intel differ enough in their x86 instructions that virtual machines can't transfer from one to the other, and they won't be able to do so for the foreseeable future. Even with a single manufacturer, the differences between chip product lines can be significant enough to halt transfer. Intel Xeon servers, for example, can't host virtual machines coming from Pentium servers.

Still, chipmakers are trying to aid live migration. The virtualization hooks they've added to their chip architectures help the hypervisor tell which x86 instruction set it's using. "Our goal is to make migration possible between as many different processors as possible," says AMD's Tim Mueting, manager for virtualization solutions.

Intel is adding FlexMigration virtualization hooks to its multicore Penryn chip line, due in the fourth quarter. If virtualization vendors take advantage of the hooks, then virtual machines will be able to migrate from a Penryn-based server to any server using Intel's dual-core or quad-core chips. However, that hot migration capability won't extend back to the Xeon and Pentium generations of Intel processors.