Thinking it through, I realised that I was using a 32-bit kernel - 2.6.18-194.26.1.el5 - which doesn't, by default, support more than 3 GB.
Therefore, I had two choices: -
- Switch to a 64-bit kernel and/or distribution, meaning that I'd potentially lose all of my installed code
- Use the Physical Address Extension (PAE) kernel
yum install kernel-PAE ( note that PAE is capitalised )
Having done this, I needed to configure it to be my default kernel. Now RHEL uses GRUB, but there's no obvious GUI to manipulate the GRUB configuration files, and I wanted to be a wimp and avoid the need to manually hack them.
Tada ... to the rescue comes system-config-boot which is a graphical tool to allow one to manage the GRUB configuration.
I installed it in the same manner: -
yum install system-config-boot
and then ran the command; this allowed me to select the right kernel - 2.6.18-194.26.1.el5PAE - and then save the new configuration as default.
When I rebooted, I was able to see all 4 GB as available for use :-)
I note that RHEL 6 has deprecated system-config-boot - sadly with no replacement :-(