Understanding RAM and Disk Space when using Windows on a Mac?
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.