VPS Servers with OpenVZ technology provides two types of server memory:
1. Guaranteed RAM
Guaranteed RAM memory is dedicated memory that your VPS server can use fully at any time.
2. Burstable RAM or vSwap
Burstable RAM (older 2.6.18 kernels)
Burstable RAM is shared memory that your VPS server may share and use, while the other VPS servers are not using it and if it is avalable. But, if other VPS servers needs this RAM, it must be freed promptly, so if your VPS server applications uses this burstable RAM it will be forced to free it, sometimes this forced RAM release can cause services malfunction and make other stability issues.
vSwap Virtual Swap (newer 2.6.32 kernels)
In the newer 2.6.32 kernels burstable RAM is called vSwap (virtual swap). This feature emulates swap functionality to a VPS server (it slows down VPS speed when programs "swaps-out" or "swaps-in"), but still uses node RAM for this purpose, because real swap is available to a node server only. There is a lot of stability problems with vSwap enabled servers.
Therefore, in planning the services and programs you will run on your VPS server, you should evaluate your memory needs and choose the right OpenVZ VPS plan. And you may have to configure your services accordingly.
And remember, there is no swap for OpenVZ VPS Servers. In our KVM VPS Servers we provide standard 256MB swap.
Tips to reduce memory utilization for MySQL and Apache services can be found here.