As if Hyper-V hadn’t attracted enough attention the latest push by Microsoft to gain traction with the hypervisor is through Dynamic Memory, and since the announcement we at dataplex have had a plethora of questions. This new capability is due to be released into SP1 for Windows 2008 R2 which is rumoured to arrive no earlier than Q4 2010.
For a lot of time Microsoft and Citrix downplayed the VMware’s memory over commitment techniques, and I guess the inclusion of “similar” technology just goes to enforce the need for it, or is it that VMware play on this lack of feature as immaturity in the product? Who knows, but it has made the product feature set.
James O’Neill, IT Pro Evangelist at Microsoft, has shared some information about it, trying to explain why Dynamic Memory is not about over commit memory:
“…CPU is naturally dynamic: the CPU switches workload to another; we can reserve a quantity of CPU for a VM and or cap the amount it gets and if total CPU demand exceeds supply, weightings mean the shares granted to each VM do not need to be equal. Network is the same: packets go in or out – the NIC works for one task at a time but multiple tasks are easily multiplexed through a single physical Network interface. Storage is different – because stuff is stored whether it is actively being processed or not, so commitment of disk space is long term. We don’t have to allocate all the disk a VM might use when we create the VM: instead of a fixed-size disk, dynamic disks can grow as needed – the sum of maximum disk sizes can be greater than disk capacity of the host, but at any given time we can’t use more disk than there is present. Now we’re applying that to memory…
VMs either have fixed size memory or a minimum , maximum and weight. So we don’t need to commit memory based on the peaks in load in the VM – dynamic memory will monitor demand for memory and use the hot-add capabilities of modern OSes to increase memory. Eventually all the memory will be committed and since you can’t hot unplug memory we have a component to take memory out of use in one VM so that it can be given to another – and Hyper-V will take memory from the VMs which need it least. Dynamic memory won’t be supported on every possible operating system…
By de-allocating memory in VMs Hyper-V ensures the total allocated remains below the total present: when a VM can’t receive any more memory the OS in it will decide which caches should be abandoned, and which pages they should swap to disk. Hyper-V never swaps VM’s memory to disk. You can have a design which over-allocates, and swaps when over-committed (and uses page sharing to allow some over-allocation before reaching the point of over-committing) or one which doesn’t swap and so doesn’t need page sharing to reduce swapping – but can’t over-allocate either…
So Dynamic Memory uses the memory ballooning technique.
Talk to us to understand more; cb@dplex.co.uk