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I recently encountered an issue where I had installed a 32-bit RHEL 6 server on a system with a 64-bit processor. I did some research and found that RHEL 6.0 has PAE enabled on 32-bit machines, which allows the user to use up to 64 GB of RAM in a 32-bit OS.

However, RHEL 6 server 32-bit with PAE still has the limitation that a single process can only have a maximum address space of up to 4 GB. These details could be seen from here and here.

So, I wish to upgrade my 32-bit installation of RHEL 6 server to a 64-bit installation. How could I do it without losing any of the data that is already present on the 32-bit machine?

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  • I think the process limit of a 32 bit process, and 32bit kernel (pae or other wise), is 3GB. 1GB is reserved by kernel for its own use. I upgraded my kernel to 64bit, now 32bit apps can use full 4GB. I just installed new kernel and told grub to use it. ( I am using debian ). – ctrl-alt-delor Jul 16 '14 at 17:59
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First things first.

I had setup my 32 bit system as below. 

/boot - /dev/sda1
/     - /dev/sda2
swap  - /dev/sda3
/home - /dev/sda4

If you have not setup the /home in a different partition, then you have to backup all the data and restore it. You cannot do as described in this answer.

/home is in different partition

So if you have /home in different partition, you can do the below steps.

  • Insert the RHEL 6.0 64 bit DVD into the drive or use an USB disk that has the image of 64 bit OS.
  • We will get the options to do a fresh installation or upgrade to another RHEL version.
  • If we select Upgrade option, it will fail with an exception because upgrading directly from 32 bit to 64 bit is not possible. Even before trying, Redhat gives a clear message saying that the process won't succeed.

  • Select the fresh installation option. Now, do the following.

    /dev/sda1 - Select the checkbox for formatting the bootloader 
            and change the mount point as /boot. 
/dev/sda2 - Select the checkbox for formatting the / directory 
            and change the mount point to /.
swap      - do nothing. 
/dev/sda4 - Select the mount point as /home and do not do anything else.

  • Proceed with the installation process as you normally do and we can have a 64 bit RHEL without affecting any of the user data.

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  • On debian you can first install 64bit kernel, then install 64bit libs, then install some 64 bit application. (I have only done the first). – ctrl-alt-delor Jul 16 '14 at 18:01

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