* Understand that disk is 300,000 times slower than memory and not an adequate substitute for it.
* Understand your VM. Particularly, if it's Java, that the JVM will eventually 'walk' the entire heap you set with -Xmx. If you have a number of VMs with massive maximum heaps, that exceeds your physical memory, you will be screwed shortly.
* Understand that disk is 300,000 times slower than memory and not an adequate substitute for it.
* Understand your VM. Particularly, if it's Java, that the JVM will eventually 'walk' the entire heap you set with -Xmx. If you have a number of VMs with massive maximum heaps, that exceeds your physical memory, you will be screwed shortly.