RHEL / CentOS Support 4GB or more RAM ( memory ) using PAE Kernel

If you have 4 GB or more RAM use the Linux kernel compiled for PAE capable machines. Your machine may not show up total 4GB ram with 32 bit architecture servers. All you have to do is install PAE kernel package.

This package includes a version of the Linux kernel with support for up to 64GB of high memory. It requires a CPU with Physical Address Extensions (PAE). The non-PAE kernel can only address up to 4GB of memory. Install the kernel-PAE package if your machine has more than 4GB of memory (>=4GB).

How Do I Install PAE kernel?

To install PAE kernel, use yum command:
# yum install kernel-PAE
# grub
grub> savedefault --default=0 --once
grub> quit
# reboot

If the kernel boots successfully edit /etc/grub.conf again to set the new kernel as the new default for the future reboots.

How to Boot second kernel if first fails?

If for example your grub.conf looks like the one i have below. This boots into the 2.6.20-1.2944 by default.


default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Fedora Core (2.6.20-1.2944.fc6)
        root (hd0,0)
        kernel /vmlinuz-2.6.20-1.2944.fc6 ro root=LABEL=/ acpi=off
        initrd /initrd-2.6.20-1.2944.fc6.img
title Fedora Core
        root (hd0,0)
        kernel /vmlinuz-2.6.20 ro root=LABEL=/ acpi=off
        initrd /initrd-2.6.20.img


If you want to have it so that 2.6.20 boots on next boot, run the following command from the console before rebooting.


echo “savedefault –default=1 –once” | grub –batch
then
reboot


Now, if you have a kernel that panics and need it to reboot if it panics. change your config and add the panic=# (# being seconds before auto reboot) to the kernel line. Example file below


default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title Fedora Core (2.6.20-1.2944.fc6)
        root (hd0,0)
        kernel /vmlinuz-2.6.20-1.2944.fc6 ro root=LABEL=/ acpi=off panic=5
        initrd /initrd-2.6.20-1.2944.fc6.img
title Fedora Core (2.6.20)
        root (hd0,0)
        kernel /vmlinuz-2.6.20 ro root=LABEL=/ acpi=off panic=5
        initrd /initrd-2.6.20.img


so the order of operations on this are as follows.


- Add new kernel to grub.conf
- set default=# in grub.conf to the failsafe kernel (the one you want it to try if the other fails)
- run the following:


echo "savedefault --default=2 --once" | grub --batch; reboot