Understanding RAM and Disk Space when using Windows on a Mac? I am trying to understand how you need to set things up when trying to run Windows from a Mac (using Bootcamp /  Parallels).  I understand the differences in terms of Bootcamp requiring you to choose at bootup versus running side by side with Parallels but I want to understand the implications around HD and RAM.

So lets say I buy a Mac with 512 GB SSD and 16 GB of RAM.  I see that if you want to install windows, you need to partition and allocation memory and disk space for Windows.

On Windows I would need to run:


SQL Server 2008
Visual Studio 2010
Microsoft Office
and a few other pieces of propriety software


So my questions are:


How do i decide how much space is allocated to Windows and once I decide that, can I change it in the future?  I was thinking about allocated 100GB to Windows which seems like it should cover me.
How do i decide what RAM is allocated to Windows.  Can I change that on the fly in the future after that decision?  Once I do this (lets say i allocate 8GB of RAM to windows), either through BootCamp or Parallels, does that mean that my Mac only will be able to leverage 8GB (since i have a total of 16GB) regardless if I am running Windows at that time or it only "takes" 8GB away when i start up windows.
If you run Parallels off a disk image, you can expand that partition to as much disk space as you have available. All it requires is shutting down the VM. As far as I know you can't shrink a disk image.


If you're running Parallels off a Boot Camp partition, it's possible in some cases to expand that partition later on, using third party tools. This depends on how the other partitions on the disk are set up (typically a partition can be expanded to fill free space after it, but you can't change the starting point on the disk).

How much RAM you allocate can be changed using Parallels. You simply shut down the VM, change the setting in Parallels and restart. This doesn't change whether you're using a disk image or a Boot Camp partition, but it's worth pointing out that if you boot from the Boot Camp partition (not in Parallels), that's running Windows directly and it will use all the RAM available to the system, like any other laptop.


The RAM allocated to your VM is only "used" when the VM is running. If you shut down Parallels, OS X can use the full amount in your system.

How you allocate disk space and RAM depends on what you're doing with your VM, and what you expect to be doing with OS X while that VM is running. Think of it as configuring a whole new computer - you need a quantity of hard drive space and RAM sufficient for the tasks at hand. The easy part is that you can (mostly) change your mind later on if you guess wrong with a VM.