The new CentOS 6 OpenVZ kernel has a new memory management model, which supersedes User beancounters. It is called VSwap.
When the Guaranteed Ram limit is reached, memory pages belonging to the container are pushed out to so called virtual swap (vswap). The difference between normal swap and vswap is that with vswap no actual disk I/O usually occurs. Instead, a container is artificially slowed down, to emulate the effect of the real swapping. Actual swap out occurs only if there is a global memory shortage on the system.
When Should I Enable VSwap?
You SHOULD enable VSwap if your host operating system is CentOS 6 or uses the 2.6.32 kernel. VSwap has been designed to replace User beancounters and is built into the CentOS 6 kernel.
When Should I NOT Enable VSwap?
Do NOT enable VSwap if you are using CentOS 5 as your host operating system. The 2.6.18 kernel does not support VSwap and probably never will.
How is VSwap Calculated?
VSwap is calculated from the amount of burst memory you add to a virtual server. For example, if you specify 512MB Ram and 1024MB Burst the VSwap would be 512MB (1024MB - 512MB = 512MB).