Excessive memory demand can cause severe performance problems for one or more VMs on an ESXi host. When ESXi is actively swapping from the memory of a VM to disk, the performance of that VM will degrade. The overhead of swapping a VM's memory to a disk can also degrade the performance of other VMs because the VM expects to be writing to RAM (speeds measured in nanoseconds), but it is unknowingly writing to disk (speeds measured in milliseconds).

The counters in vSphere Client for monitoring the swapping activity are as follows:

• Memory Swap In Rate: The rate at which memory is being swapped in from the disk.
• Memory Swap Out Rate: The rate at which memory is being swapped out to the disk.
• Swapped: The total amount of data that is sitting inside the .vswp hypervisor-level swap file. However, this doesn't tell you anything about the current state of the performance, nor about the current state of free pRAM. It just tells you that at some point in the past, there was low free pRAM. The only use of this counter is to check whether there was memory shortage in the past and whether there's a risk of Swap In (and bad performance) in future. But it's hard to estimate how likely the Swap In process will be because sometimes there's junk data that will be rarely requested again (for example, pages holding rarely accessed data, such as overly aggressive filesystem caching, or zeroed out pages that were actually deemed free by the guest operating system).

High values of the counter indicate insufficient memory and that performance is suffering as a result.

However, a high Swap Out rate means that the host is low on free pRAM, but essentially it does not indicate a current performance problem. A high Swap In rate indicates a current performance problem but not necessarily that the host is currently low on free pRAM.