Roger Howorth
Roger Howorth is a journalist and IT contractor working for various clients operating in financial services.
Wednesday 17 September 2008, 9:34 PM
A better way to migrate virtual machines
VMware has been in business for 10 years, and for most of that time it based its business on the fact that Intel and AMD did not support virtualisation. Although the chip vendors both added virtualisation support to their chips a few years ago, this was only the first step. Earlier this week Intel announced new processors that include a feature called VT FlexMigration, which will help administrators move virtual machines from one host server to another without any downtime.
The ability to move a VM from one server to another without any interruption of service is one of the crown jewels of VMware’s product line. Dubbed VMotion, this is a trick that Microsoft’s Hyper-V has yet to manage. However, even VMware’s VMotion cannot move virtual machines around willy nilly. For example, VMotion can’t be used to move a VM between an Intel based server and one fitted with AMD chips. Likewise, moving VMs between different generation Intel chips is sometimes impossible.
Intel’s VT FlexMigration is all about removing some of these limitations to VMotion.
The problem with VMotion is that while all x86 compatible chips support the same basic instruction set, chips from different vendors, and even the various chip families from the same vendor, each have a different set of additional instructions. For example, Pentium 4 chips have the SSE2 instruction set. The Merom Core architecture added SSE3, and later Intel chips added SSE4 and SSE4.2. While it’s possible to VMotion a VM from an older chip to a newer one, sending it back the other way is likely to make it crash.
This is because when software starts up it is likely to check for the presence of these extra instructions. If the instructions are then taken away because the VM has been moved to a host with older CPUs, the software will crash when it next tries to use them. It’s important to note that these additional instructions could be used by a VM operating system and by its applications. Management tools like VMware VirtualCenter can mask these instructions so they can’t be used by a VM’s operating system, but without FlexMigration there is no way to hide the instructions from a VM’s applications.
Intel’s VT FlexMigration provides a way for management consoles like VMware VirtualCenter to hide these additional instructions from a VM so that it doesn’t try to use them. In turn this means that server administrators can move their VMs around between a wider range of server hardware, which gives them more flexibility in how they allocate VMs to host servers.
The ability to move a VM from one server to another without any interruption of service is one of the crown jewels of VMware’s product line. Dubbed VMotion, this is a trick that Microsoft’s Hyper-V has yet to manage. However, even VMware’s VMotion cannot move virtual machines around willy nilly. For example, VMotion can’t be used to move a VM between an Intel based server and one fitted with AMD chips. Likewise, moving VMs between different generation Intel chips is sometimes impossible.
Intel’s VT FlexMigration is all about removing some of these limitations to VMotion.
The problem with VMotion is that while all x86 compatible chips support the same basic instruction set, chips from different vendors, and even the various chip families from the same vendor, each have a different set of additional instructions. For example, Pentium 4 chips have the SSE2 instruction set. The Merom Core architecture added SSE3, and later Intel chips added SSE4 and SSE4.2. While it’s possible to VMotion a VM from an older chip to a newer one, sending it back the other way is likely to make it crash.
This is because when software starts up it is likely to check for the presence of these extra instructions. If the instructions are then taken away because the VM has been moved to a host with older CPUs, the software will crash when it next tries to use them. It’s important to note that these additional instructions could be used by a VM operating system and by its applications. Management tools like VMware VirtualCenter can mask these instructions so they can’t be used by a VM’s operating system, but without FlexMigration there is no way to hide the instructions from a VM’s applications.
Intel’s VT FlexMigration provides a way for management consoles like VMware VirtualCenter to hide these additional instructions from a VM so that it doesn’t try to use them. In turn this means that server administrators can move their VMs around between a wider range of server hardware, which gives them more flexibility in how they allocate VMs to host servers.
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