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Hyper-V Service Pack To Add Dynamic Memory

Last week, Microsoft announced a number of new virtualization technologies targeted at the desktop space. The additions are primarily focused on virtualized desktops, VDI and application virtualization. Somewhat of a footnote to all of this is that a new addition, dynamic memory, will apply to all Hyper-V virtualization, not just the desktop technologies.

Dynamic memory was featured at one point in the Hyper-V R2 product, but has since been rescinded as of the current offering. Its whereabouts has been a hot topic on VMware employee Eric Gray's vcritical blog and made rounds in the blogosphere.

The dynamic memory feature will apply to all Hyper-V workloads. It will work with a basic principle of a starting and a maximum allocation. The simple practice is to make the starting allocation the base requirement for the OS in question -- 1 GB for Windows 7 for example -- which is counter to what VMware's virtualization memory management technologies provides.

I had someone explain dynamic memory's benefits to me and the simplified version is that dynamic memory is an extension of the hot-add feature to allow more memory to be assigned to a virtual machine. Cool, right? Well, maybe for some and only if we think about it really hard. Consider the guest operating systems that support hot-add of memory. Doing some quick checking around, I end up with Windows Server 2008 (most versions), Windows 7 and Windows Server 2003 (Datacenter and Enterprise only).

The main thing that the enhanced dynamic memory will address is the lack of hot remove, which is a good thing. Basically, it is easy to do a hot-add, but what if you want to step it down after the need has passed? This is where the R2 features will kick in and reclaim the memory. I don't want to call dynamic memory a balloon driver, but it will automagically mark large blocks of memory as unavailable, which in turn will allow that memory be reclaimed back to the host.

One fundamental truth to take away is that Hyper-V will never allocate more than the physical memory amount. Any disk swapping won't take place in lieu of direct memory allocation.

Given that we don't have public betas yet, a great deal of detail still needs to be hammered out. Make no mistake: Microsoft's Hyper-V virtualization takes an important step in the right direction with the dynamic memory feature. It is not feature-for-feature on par with VMware's memory management technologies, but this increased feature set has sparked my interest.

Stay tuned for more on this. I and others will surely have more to say about dynamic memory.

Posted by Rick Vanover on 03/23/2010 at 12:47 PM


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