If you are using vCenter Server and plan on upgrading to vSphere you should be aware that vCenter Server 4.0 uses much more memory than vCenter Server 2.5 does. According to the official documentation for both 2.5 and 4.0, the system requirements are very similar:
- 2.0 GHz or faster Intel or AMD x86 processor. Processor requirements can be more if your database runs on the same hardware.
- 2 GB RAM. You might need more RAM if your database runs on the same hardware.
But VMware has a note in the vSphere documentation that states:
vCenter Server includes a service called VMware vCenter Management Webservices. This service requires128MB to 1.5GB of additional memory. The vCenter Management Webservices process allocates the required memory at startup.
While the documentation lists equal memory requirements for each version you will find that vCenter Server 2.5 typically uses very little RAM. My tomcat5.exe service on a vCenter Server 2.5 server that manages 6 ESX hosts has been running for weeks and is only using about 50 MB of RAM. The total RAM usage for the host is less than 600 MB. On my vCenter Server 4.0 that has been running for a few days and manages 2 ESX hosts, tomcat6.exe is using almost 1.2 GB and the total RAM usage is 1.5 GB.
So while the memory requirement for both versions of vCenter Server may be equal (2 GB) the actual memory usage between the two versions is not. In version 2.5 you could easily get away with 1 GB, but for version 4.0 you definitely need 2 GB.