Idle VMs - Why should we care? (2 of 3)

In Wednesday's blog I mentioned the term VM Sprawl and this  is where Idle VMs are likely to factor. 

Often VMs are provisioned to support short term projects,  for development/test processes or for applications which have now been decommissioned.  Now idle, they’re left alone, not bothering anyone and therefore not on the Capacity and Performance teams radar.

Which brings us back to the question.  Idle VMs - Why should we care? 

We should care, for a number of reasons but let's start with the impact on CPU utilization.

 When VMs are powered on and running, timer interrupts have to be delivered from the host CPU to the VM.  The total number of timer interrupts being delivered depends on the following factors:

·         VMs running symmetric multiprocessing (SMP), hardware abstraction layers (HALs)/kernels require more timer interrupts than those running Uniprocessor HALs/Kernels.

·         How many virtual CPUs (vCPUs) the VM has.

Delivering many virtual timer interrupts can negatively impact on the performance of the VM and can also increase host CPU consumption.  This can be mitigated however, by reducing the number of vCPUs which reduces the timer interrupts and also the amount of co-scheduling overhead (check CPU Ready Time). 

Then there's the Memory management of Idle VMs.  Each powered on VM incurs Memory Overhead.   The Memory Overhead includes space reserved for the VM frame buffer and various virtualization data structures, such as Shadow Page Tables (using Software Virtualization) or Nested Page Tables (using Hardware Virtualization).  This also depends on the number of vCPUs and the configured memory granted to the VM.

We’ll have a look at a few more reasons to care on Monday. In the meantime sign up and come along to my VMware Memory webinar next week http://www.metron-athene.com/services/training/webinars/index.html

Jamie Baker

Principal Consultant