Should I use 32-bit or 64-bit OS for my CentOS server running small VMs?

Nickolai Leschov asked:

I’m going to run CentOS 6 on Dell PowerEdge R210 II with Xeon E3-1220 CPU and 4 GB RAM.

I’m going to run several virtual machines on it, running various setups of websites. VMs most certainly be 32 bits, to conserve memory and memory bandwidth; nothing inside of them can be that large.

Now I’m inclined to have the host OS be 32-bit too, but I’m thinking whether or not I might be better off installing 64-bit “just in case”. I may or may not upgrade the RAM over the lifetime of this server (my previous server had 1 GB over its whole lifetime), but I know Linux supports PAE well and up to 64 GB of RAM should be accessible over PAE. I’m running 32-bit Ubuntu on my 64-bit laptop with 8 GB just fine (it uses less RAM and I think boots faster). And I know the individual processes and VMs won’t get too big as to run into a limit of 32-bit system.

What do you think: should I run 64-bit host “just in case”?

My answer:

Use 64-bit to preserve an upgrade path. RHEL 7 will only be available in 64-bit, so if you use 32-bit, you will not be able to upgrade and will have to do a fresh installation when you want to move to the new version of the distro.

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