Theses notes talk about the hypervisor in Hyper-V and compare it to how other hypervisors in the marketplace manage memory - Many Thanks to the source Hypervoria.com for theses details.

 

- Monolithic versus Microkernel
Hyper-V is based on a microkernel hypervisor, where there is no shared memory pool amongst virtual machines. This isolates virtual machines so that security problems like buffer overflows attacks do not traverse to other virtual machines on the same underlying host.

 

This is not just a theoretical security concern, look at CVE-2008-4917. VMware uses a Monolithic hypervisor where there is a shared memory pool that all virtual machines draw from. The tradeoff is scalability for performance, as virtual machines are allowed to overcommit, for example if the physical server had 32GB of RAM, VMware lets you run 64 virtual machines with 1GB of RAM each.

 

- What Dynamic Memory is not.
Microsoft did not change the hypervisor from Microkernel to Monolithic in 2008 R2 SP1. Dynamic memory in 2008 SP1 is nothing like VMware’s memory overcommit.  It simply states “Microsoft Dynamic Memory will allow customers to adjust memory of a guest virtual machine on demand to maximize server hardware use.”

 

- What Dynamic Memory will actually allow
According to one commenter on the Windows Virtualization Team blog “Microsoft’s implementation of Dynamic memory is allocating ram automatically among guest depending on usage. You still can't allocate more RAM than what you have physically.”

 

Dynamic Memory was first announced at PDC in 2008, and at that time, the way VM memory configuration was going to work was:

 

- Initial (what VM will boot with)
- Minimum (what VM is guaranteed)
- Maximum (what VM can grow to)
- Memory is added via Hot-Add MEM functionality
- Memory is removed via Balloon driver (on supported OS only)

 

Let’s create a scenario where this would be beneficial. Imagine a host server with 64GB of RAM, and 8 VM’s each configured with 8GB of ram maximum, but only using 6GB (this would be the minimum memory setting on the VMs). That leaves 16GB of RAM available on the host.

 

In 2008 R1 and 2008 R2 RTM, that memory could not be touched. With Dynamic memory in 2008 R2 SP1, this memory could be used for new or existing virtual machines.