Best Practices: Viewing virtualization from every angle


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Virtualization isn't just VMware
When we talk about virtualization, the first name that comes to mind is VMware. While VMware is the leader in this space with its ESX Server, it isn't alone. AIX shops will more than likely be familiar with IBM's logical partitions (commonly known as LPARs) that can run AIX and Linux. There's also Microsoft's Virtual Server and PC, and Sun with its Solaris Zones. And the Open Source movement isn't far behind with Xen.

When people talk about server virtualization, ask for specifics. What's the rationale behind their selection? More importantly, is it supported on your SAN/storage environment with minimal/no changes or are costly modifications/ additions necessary to make it work?

A new take on sharing IO
While it's true that in most cases the connectivity to the guest operating system is directly or indirectly via the hypervisor itself, don't assume that it's always the case. In some types of systems, the guest operating system can have access to its own independent host bus adapters (HBAs). In those situations, you have to work with the system teams to figure out which guests have direct access and which share IO via the hypervisor.

Then there's the issue of how the LUNs are presented and mapped from the hypervisor to the guest. For example, IBM has introduced VIO, in which the hypervisor

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(or VIO server) can present LUNs directly via a virtual SCSI adapter to the LPAR. The VIO server knows nothing about how the LPAR uses these LUNs. Another way of doing it is to create logical volumes from the VIO server or hypervisor, and then present those to the LPAR instead of raw disks. This not only gives you visibility and control over how IO is distributed but, by way of the logical volume manager (LVM), it minimizes performance bottlenecks. So you have two ways to do it. The first way is simpler but results in more administrative overhead because disk numbers can change in AIX for any number of reasons. The latter way, using LVM, allows all of the presentation to be controlled by an underlying layer that controls disk IDs.

To me, how LUNs are presented and IO is shared between the virtual servers is a critical design issue. This is where the most attention should be paid to create a healthy and scalable virtual environment.

This was first published in December 2007

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