Pivot Point

My recent series of blog articles have discussed ESX memory management the the performance specter of host swapping. My last article attempts to correct the misconception that VMware recommends against over-commit memory.  In that article I suggested that memory over-commit is requirement in optimizing memory utilization. Today I want to provide a specific example to show why this is true.   I am have also included tips for identifying host swapping in your environments.

Understanding the Bottleneck

Let me show the value of over-commit and danger of swapping by way of an example. I will choose the following typical values to demonstrate my point:

• All virtual machines are on a single host which has 32 GB of RAM installed.
• Each virtual machine is sized to 8 GB of RAM.
• Each virtual machine has 25% active memory (%ACTV in esxtop and “Active” in vCenter).

A virtual machine’s active memory is dictated by the application and its usage.  But the VI admin has complete control over the number of virtual machines in the environment which means host active memory can be influenced by adding or removing virtual machines.  Because virtual machine active memory is always equal to or less than 100% the only way to drive the host active memory to 100% is to over-commit memory.  This is why hypervisors that do not support memory over-commit are simply not viable for data centers where memory optimization is a priority.

Identifying and Correcting the Bottleneck

The ongoing occurrence of swapping is identified by a non-zero swap rate in either esxtop or vCenter.  In addition to swap rate, esxtop provides a swap wait time in its CPU panel.  When swap rate exceeds hundreds of kilobytes per second or swap wait time exceeds a couple percentage points, it is time for corrective action.

There are three possible solutions to this problem:

1. Balance the virtual machines’ memory usage by moving virtual machines from hosts with higher amounts of memory usage to hosts with lower amount of memory usage.
2. Run fewer virtual machines.
3. Buy more memory.

Designing Your Infrastructure to Simplify Memory Management

Ultimately I owe you a full white paper on memory management to provide a sufficient answer. But I want to give you two ideas of the tools and techniques that I will be describing when in this future paper.  First, place host swap files on solid state disk (SSD) stores to improve their performance.  With the right SSD device it may be possible to eliminate swap penalties.  Second, even if SSDs are unavailable consider consolidating multiple swap files onto a single store.  This will make swap rate monitoring very easy but may compound the performance penalties of swapping.

Stay tuned and VMware will provide more documentation on memory management in 2010.