VMware ESX Memory Over Commit Technology Explained
January 30th, 2009 | 
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Jason Boche’s post titled Idle Memory Tax is a great read if you are trying to understand ESX memory allocation between virtual machines (VMs). Specifically, the post does a great job explaining how it works when you over commit your physical host’s memory. In other words, the sum of all the RAM assigned to the VMs running on a host is greater than the actual physical RAM of the ESX server.
Here’s a quote from Jason that briefly explains part of the technology that makes over commit possible: the Idle Memory Tax (IMT).
“Quite simply it’s a mechanism to take idle/unused memory from guest VMs that are hogging it in order to give that memory to another VM where it’s more badly needed. Sort of like Robin Hood for VI. By default this is performed using VMware’s balloon driver which is the more optimal of the two available methods. Out of the box, the amount of idle memory that will be reclaimed is 75% as configured by Mem.IdleTax under advanced host configuration. The VMKernel polls for idle memory in guest VMs every 60 seconds.”
Read the entire post for much more technical details and examples.
I’ve blogged before about the symptoms when the IMT and the ESX balloon driver can no longer keep up and it’s time to add another ESX host and spread the VM load.
I believe that ESX 3.x changed the need to
over commit resources on a single host. Once an ESX Cluster was possible, DRS could proactively minimize resource conflicts and extra capacity was needed for HA, so VMware VI capacity design shifted to the N+1 approach and administrators now focus less on Resource Pools and Shares. Today, most ESX hosts are utilized well below 70% of their resources.
Even though ESX host over commit was more prevalent in ESX 2.x implementations, with today’s economy and IT budget cuts maybe stand alone implementations of ESX/ESXi is a trend on the rise. VMware’s unique IMT and balloon features can help you consolidate more servers then some of the other virtualization server alternatives.
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