OpenVZ containers allow us to set 2 types of RAM, guaranteed and burstable.
Burstable RAM is a temporary backup of resources over your standard memory quota. This can be offered to users of Virtual Private Servers. Each VPS has a guaranteed amount of RAM that it's able to use at most of the times. It is possible that the usage of web service may exceed the allotted quantity of guaranteed RAM available to us, this is when the backup RAM comes into play. It is provided to the user during times of maximum usage or traffic congestion. This RAM is not in use by any VPS on a server and can be allocated to any VPS that may need it as long as it is available. So as long as those resources are available, you as a user will be able to go over that limit of guaranteed RAM.
Burstable RAM is built to handle accidental additional requests. If you are getting more numbers of requests and your current web server cannot handle the extra load then you need to shift to more powerful resources.
