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A client in the Support Practice Group recently ran into an interesting problem wherein a virtual machine outgrew its LUN (or logical disk) and lost the ability to boot.  While in most virtualized environments this might seem like an easy issue to resolve or to avoid entirely for that matter, a clustered Hyper-V Server 2008 environment that has LUNs carved from an HP MSA disk array suffers from limitations that may not be particularly obvious during initial network planning.

The first major limitation of this network design results from the way in which Hyper-V Server 2008 supports high-availability.  While the introduction of Clustered Shared Volumes in Hyper-V Server 2008 R2 allows many virtual machines to exist on the same LUN and still be highly available, the Server 2008 release requires a dedicated LUN for each highly available virtual machine.

A related issue derives from the fact that an HP MSA smart disk array can contain, at most, 32 LUNs per device.  To maximize both useable storage space and the number of virtual machines that can be built, it makes sense in a Hyper-V Server 2008 environment to carve out as many LUNs as possible on day one and to combine the leftover space on the array into one larger utility LUN.  Unfortunately, in an HP MSA only the last logical disk created can be deleted.  Thus expanding an intermediate LUN after a network has been set up in this fashion would involve first deleting the utility LUN in order to create space.  The result is a set of LUNs that cannot be easily or painlessly expanded.
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I’ve seen a lot of talk lately about VMware’s Transparent Page Sharing (TPS) and how it is affected by ASLR in Windows 2008/Windows 7. I wanted to see if there was any real measurable reduction in shared memory when using ASLR vs. when it was disabled. First, let’s talk about what TPS and ASLR actually are and what the acronyms mean.

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