Windows 7 - XP Mode VM Memory Allocation recommendation?

I'm using Windows 7 Pro x64 running on an i7-2600K @ 3.5GHz with 16GB @ 1600MHz with an Intel X25-M 160GB SSD (or in other words a fast machine with a fast hard drive with heaps 'o memory. Having so much main memory (which is cheap these days so why not) I've got no paging file at all, which I personally think is a good idea (please tell me I'm wrong if I am).

My question is about how much memory I should allocate to my "XP Mode" virtual machine, and whether I should use a paging file inside that VM. My own personal logic is to assign it 3GB (the max XP-32 can deal with) and as I've given it that much, I can disable it's page file too. That seems to me to be the ideal configuration. I'm hearing from others though that the load on the host OS by having to handle that much memory for one of it's VMs is more than the benefit gained by the VM not having to have a paging file. I'm being told I should allocate somewhere between 512MB an 1GB to the VM and let it use a paging file when it needs to. That seems counter-intuitive to me.

My host OS/Processor is way under-utilized most of the time and can easily handle any extra hard work it needs to in order to provide the VM that much memory. Plus not having to write to a virtual paging file which in turn would be written to a real VHD on a real drive can't be better than using memory. Can it?

XP x86 can see and use 4GB - you're confusing virtual address space with memory. A 32bit process can use maximum 3GB of Virtual Address space (VA) (with the /3GB boot.ini switch in XP/2003 or the IncreaseUserVa bcdedit param in Vista or Windows 7), whereas a 32bit operating system (without PAE capabilities, like all 32bit Windows desktop OSes) can use a maximum of 4GB of RAM. This does not change if it is a virtual machine, so if you have heaps of memory just sitting idle, assign 4GB to the VM.

As to running with a paging file or not, with 16GB of RAM that is your choice. I'd still recommend 512MB - 1GB if you want to be able to capture a crash dump properly if the system were to ever crash on you, but if that's not important (and you aren't running enough programs and VMs to commit more than 16GB of RAM at any one time) then you will be fine.

Thanks for that, you learn something new every day I don't think I'd bother giving it 4GB, I imagine 2GB is easily enough and still not need a page file. It's not the only memory hog on my machine though, I'm often running a Vista and a Win 7 VM too (and they have 3GB each), plus I have some memory hungry apps on the host too.

But if you didn't care about capturing a crash (and I don't at all) then you'd still go with more, and no page file? This VM's VHD file gets backed up daily, so any installations, changes or windows updates that break it will very easily be backed out.

I personally run with no paging file, and I have no machines with less than 16GB of RAM (I've actually started going with 32GB workstations for my personal machines, running Server 2008 R2, and laptops with 16GB of RAM running Windows 7). I used a paging file with 8GB of RAM, but I know my own habits and have analyzed via perfmon what memory requirements those are (including VM load), so I knew there could potentially be times when I'd commit 9 or 10GB of VA to memory - they would be very infrequent, but that would cause a bugcheck without a paging file. Once I moved up to 32/16 on all my machines, I do know for sure that I don't need a paging file, and as such I don't run with one.