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If answer is negative then which workaround would be less painful? Reinstall the system? Run mongo in virtual machine with 64bit ubuntu?

For now I need mongo only for dev purposes, but my test database consists of 3kk entities so I'm afraid virtual machine will be inefficient.

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According to the KVM FAQ while it is possible to run 32-bit guests on top of a 64-bit host, "the only unsupported combination is a 64-bit guest on a 32-bit host". So virtualisation will probably not help you in this case.

The PAE extensions really only help if you are running many processes, each of which need less than 4GB of address space. While the extension allows the operating system to manage more memory, it is still limited by the 32-bit address space.

If you have a single memory hungry process you really need a larger address space, which you get through a 64-bit kernel.

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The actual problem is that 32bit mongo supports only small (up 2Gb) databases. Everything else is just fine. Regarding visualization: is the above-mentioned limit KVM specific or is it a general rule? –  Moonwalker Feb 21 '12 at 4:03
    
Right. The 2GB limit is because MongoDB memory maps its databases into the virtual address space: if you have a small virtual address space, then it limits the size of your database. I have seen some claims that VMware Server could run 64-bit guests on a 32-bit host, but I'm not sure how they'd do this efficiently when the CPU is running in legacy mode (especially once you started to address more than 4GB of virtual memory). –  James Henstridge Feb 21 '12 at 4:36
    
Thank you! So it seems that my only solution is reinstall? Is there a way to "upgrade" ubuntu from 32bit to 64bit? For example change sources.list and do some other tricks? –  Moonwalker Feb 21 '12 at 8:59
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There are ways to switch an install (e.g. see askubuntu.com/questions/81824), but they are non trivial. Reinstalling may be your best bet, and can be done while leaving your home directory intact. The software center also has a facility to upload the list of additional packages you've installed, which might be helpful in reconstructing the system on the new architecture. –  James Henstridge Feb 21 '12 at 9:50
    
Thank you very much! –  Moonwalker Feb 21 '12 at 10:37

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