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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"?

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  • 5
    Just use 64-bit. – ewwhite Mar 26 '14 at 13:12
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The only time you should care about whether your OS is 32-bit or 64-bit is when you have an application that specifically needs one of the two.

If you do not have such an application requirement, and/or you have more than 3.2 GB of memory, you may as well just use 64-bit.

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  • Where does 3.2 GB figure come from? I know it's true for 32-bit Windows, but in CentOS - I think that's not a limit. I know for a fact that 32-bit Ubuntu installs PAE by default and utilizes my 8 GB RAM just fine. According to this article, Linux should be able to utilize 64 GB. – Nickolai Leschov Mar 26 '14 at 13:15
  • @NickolaiLeschov The limit everywhere is 4gb - MOST LIKELY minus some memory for hardware. This is totally bios / chipset dependent. And on older machines.... well... there is a limit way below 4gb. On most machines these days this stuff is mapped so all 4gb can be used. Unless one has a cheap shared memory graphics card / chip. PAE is a LOT slower than linear memory access, so on your latptop you give up speed - may or may not be an issue, depending on application, but no real gain to run the OS on 32 bit. – TomTom Mar 26 '14 at 13:33
  • Many 32bit OSes can use more than 2^32 byte of RAM in total, but still only 2^32 byte per process. – Philipp Mar 26 '14 at 15:23
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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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According to this page, both CentOS 5 and 6 x86 support up to 16GB of RAM. It all depends on whether or not you plan to upgrade beyond this at any point during the host's lifetime.

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