Understanding VMware Capacity - Measuring Memory Capacity (6 of 10)

Today we'll take a look at measuring memory capacity, below is a typical graph we might use to understand how memory is being used by a VM. 

The VM has been granted about 4GB RAM. That’s what the OS thinks it has to use.

The host memory in use, shows how much physical RAM is currently being used by the VM. We can see that at 09:00 this increases and at the same time shared memory reduces.

Memory Used is our term for Active memory, and remains steady throughout, as does the Memory Overhead on the host used to support this VM. What we can see is that only about 400MB of memory is being accessed on a regular basis. Between 1.5GB and 2GB of memory is not unique to this VM and is shared with others.

Balloon Driver

The balloon driver takes about 2.5GB of space in the memory of the VM. This will cause the OS to swap out to disk until it cannot do any more, and then the hypervisor starts to swap out memory also. At that point, performance is likely to be impacted.

Here's an example of the balloon driver in use.

Where ballooning is persistent (rather than an occasional spike), then changes should be made in order to ensure there is enough RAM available for the VMs in the cluster. Ballooning itself has an overhead on the hypervisor and as such there is the potential to impact performance for the host. Changes don’t necessarily have to be more RAM installed. The very first thing to consider is if any VMs have been created and “forgotten” about. On Monday I'll take a look at Cluster memory.

Phil Bell

Consultant