How to enable RAM more than 4 GB on CentOS 5.

After I had increased the memory in my server from 2 GB to 4 GB, I found my server could see the memory about 3.5 GB only. I found this problem happened from my Kernel didn’t support so I would like to upgrade the Kernel so as to use my memory at 4 GB.

And these are steps for solve this problem.


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1. Upgrade the Kernel.

# yum install kernel-PAE

2. If you would like to compile some software such as VMWare, you should install kernel-PAE-devel too. (Optional)

# yum install kernel-PAE-devel

3. Change DEFAULTKERNEL=kernel-PAE in file /etc/sysconfig/kernel such as

# UPDATEDEFAULT specifies if new-kernel-pkg should make
# new kernels the default
UPDATEDEFAULT=yes

# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel-PAE

4. Reboot

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