Why are some systems certified to run RHEL 5 with more RAM than is listed as being supported?

Issue

• Some systems certified on Red Hat Enterprise Linux 5 (RHEL 5) have been certified with 1TB of RAM, which is greater than the amount listed as supported under RHEL 5 on the supported limits page http://www.redhat.com/rhel/compare/.

Environment

• Red Hat Enterprise Linux 5
• Systems certified with 1TB of RAM

Resolution

These systems were certified after extensive in-house testing with 1TB of RAM to ensure that operating beyond the published limits would not cause any issues. Note that there is a 40-bit limit to memory addressing in 64-bit RHEL 5. This means that the maximum amount of RAM that can be addressed cannot exceed 1 TB. Therefore, if the BIOS reserves any memory, the operating system may not see a full 1TB of RAM.