W2008 R2 SP1 Dynamic Memory Explained

Dynamic Memory can be a little confusing at first so I thought I’d give it a go.  It’s a new feature in Windows Server 2008 R2 Service Pack 1, the beta of which will appear some time in July 2010.  Dynamic Memory will support virtual machines running:

• Windows Server 2003, 2003 R2, 2008, and 2008 R2 Web, Standard, Enterprise, and Datacenter editions.
• Windows Vista/7 Ultimate and Enterprise editions.

The process is a mixture of memory hot-add and ballooning.  First …

Each VM will have a bunch of settings:

• Do you want statically defined or dynamic memory for the VM?
• What will be the minimum amount of memory in the VM?
• What will be the maximum amount of memory in the VM?
• How much free memory/buffer should the VM have?

OK.  Now the tricky bit.  How does it work?

The VM will boot up with the minimum amount of RAM.  Let’s say this is 1GB.  As the VM’s requirements grow, a VSC (Virtual Server Client) driver running in kernel mode in the VM will pull in memory from the host and supply it to the VM.  This consumes more physical memory from the host.

It is important to note that memory doesn’t magically appear from thin air.  The host must have the memory that is required by the VM – otherwise we get into a nasty performance situation.

What happens on the way down when the memory requirements of the VM reduce?

As the VM no longer needs memory, the ballooning process kicks in.  Memory cannot be physically removed from the VM.  Windows wouldn’t like that!  Instead, the VSC “tricks” it.  This driver simultaneously:

• Reports to the VM that it is consuming the RAM that is being freed up.  It isn’t really.  But this prevents Windows in the VM from trying to allocate those blocks of memory that won’t really be there.  This is the balloon.
• The VSC returns the free/ballooned memory to the host so that it can be made available for other memory hungry VM’s.

As you can see in some of the demo’s that I’ve linked to recently, it’s a pretty simple, rapid and easy thing to use.

The trick here is not to abuse what Dynamic Memory can do.  There’s no point in over committing host servers.  If you know that you have an average of 75% memory utilisation across your VM’s then don’t try to get twice as much out of the host.

I think the sizing of the settings will be tricky.  I think OpsMgr reporting will prove very useful in figuring out what is best to do and how to configure the settings.

Where will Dynamic Memory be useful?

• VDI: No doubt!  The big cost here are the GB’s of RAM and that’s usually the bottleneck in a host, not the CPU.
• VM Sizing: Just like in the physical world, it’s hard to accurately size memory for VM’s.  Software vendors can be very conservative with requirements and you can end up with too much RAM in a VM.  Now you can set a range and let the VM consume what it really needs.
• Labs: We usually have limited budgets so squeezing that little bit more for a couple extra VM’s will be very nice.  I wish I could do this right now for my book lab!!!

As the Service Pack 1 beta and RC releases develop, I’m sure MS will release more information about Dynamic Memory and engineering recommendations.