What is guaranteed RAM, burstable memory, vSWAP and SWAP?

Guaranteed RAM is the RAM size which is exclusively allocated to a VPS and can be used whenever this VPS requires it.

Burstable memory basically means extra RAM which can be temporarily added to the VPS in case of the RAM shortage. We used to allocate burstable memory to our Open-VZ VPSs but later decided to switch to vSWAP. The main drawback of the burstable memory technology is that it can be taken from one VPS once other virtual machine needs to use its guaranteed RAM limit. It may result in the server pending or in the program crash. Also the allocation of burstable memory depends on the physical carrier usage namely on how many VPSs are run on it. So some virtual machines may cost the same price but show different performance.

vSWAP is an additional RAM size which is a part of the physical carrier RAM resources. This RAM may be allocated to the VPSs in case there is sufficient amount of free RAM on the physical server. Allocation of vSWAP is not guaranteed as there might be periods when every VPS which is run on the physical carrier consumes its guaranteed RAM size thus leaving no or little free RAM to be used as vSWAP by the virtual machines that need it. Usually vSWAP is used by less active applications than those which use guaranteed RAM. In comparison with burstable RAM vSWAP is more reliable as the data moved to vSWAP cannot be lost and thus the applications don’t crash. For our OpenVZ VPSs the limit of vSWAP equals to the size of the guaranteed RAM. So if the VPS has 512 MB of RAM that means it can get up to 512 MB of vSWAP.

SWAP is a hard disk partition which can be used for data storage when the system runs out of free RAM. Usually SWAP is used by less active applications. For our Xen VPSs the limit of SWAP equals to the size of the guaranteed RAM.

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