One of VMware's memory over subscribe technologies is Memory Idle Tax.
• Memory has Shares - Like reservations, a VM also has an associated number of shares. The more shares, the more priority it has over the resource if there is contention.
If a virtual machine is not actively using its currently allocated memory, ESX Server charges a memory tax — more for idle memory than for memory that is in use. That is, the idle memory counts more towards the share allocation than memory in use
• Memory Tax associates a value to each page used
• Default Idle Tax rate is 75%
• This makes idle memory cost 4 times as many shares as active memory
The end result is that VMs holding onto a lot of idle memory, will be more likely to have the balloon driver inflate inside them to try and release some of that idle memory for use by other VMs.
So put simply it's a mechanism to take idle or unused memory from guest VMs that are hogging it in order to give that memory to another VM where it’s more badly needed.
On Wednesday I'll be looking at CPU oversubscription, in the meantime register for our next webinar 'Hardware's a Commodity - Why bother managing capacity?'