VPS Servers with OpenVZ technology provides two types of server memory
What is Guaranteed RAM?
Guaranteed RAM memory is dedicated memory that your VPS server can use fully at any time
Burstable RAM or vSwap
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
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.
then, in planning all the services and programs you will run on the VPS server, you should evaluate your memory needs the right plan when you chooseVPS. And you may have to configure your services and programs accordingly
the Difference Between “Guaranteed” and “Burstable” RAM on OpenVZ
The Guaranteed RAM is the amount the can be used
the Burstable RAM is the amount that can be allocated. Generally assume that the program will allocate double the RAM it uses.