RHEL 5 backward compatibility with RHEL 4
 

I would think this question easy to answer, however I haven't found a clear answer... maybe the question is too easy.

Here is blurb I received from my "tech support person".
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"Unfortunately the actual server running PP2.0 is not fully compatible with the Red Hat Linux version required for the software.



The server IBM pSeries 630 (7028-6C4) is certified for Red Hat Enterprise Linux 5 and above but the software requires RedHat® Enterprise Linux™4 Upgrade 4"
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Excuse the oversimplification, but wouldn't Linux 5 be backwards compatible? Ie, if it needs RHEL 4, wouldn't RHEL 5 be just fine?


Thanks for your help to the uninitiated...

Not necessarily especially where drivers (and kernel modules) are concerned. As you might imagine having drivers built in to support every conceivable hardware configuration would cause the kernel to be huge and take up most of your memory.

Instead as Linux kernels mature they go to what is "generally" used and expect you to deal with special cases. A couple of examples:
1) Older kernels had built in support for early Dell PERC (SCSI RAID Adapters) and would automatcially discover them. Newer kernels still have LEGACY support available for these older cards but you have to tell it during the install that you want to use the LEGACY driver instead of the default one or it simply won't find your adapter.
2) Fibre adapters for SAN - These are commercial adapters so you typically have to get information from the vendor that made them as to what to install and save to make them work.

Since both RHEL4 and RHEL5 are based on the 2.6 kernel they are both more compatible with each other than say RHEL4 would be with RHEL3 (as the latter had the 2.4 kernel). However, there are subverion differences of the 2.6.x kernels that make them different.

Additionally there are methods of doing things that evolve over time not just with the kernel. For example when I installed Oracle CRS (RAC) on RHEL5 Oracle had a guide for RHEL4 but not for RHEL5 even though it was certified on both. I was able to make it work on RHEL5 but found many differences in how I had to do the implementation on RHEL5 that diverged with how it would have been on RHEL4.

All that seems clear as mud I'm sure. Generally I'd say if it works with RHEL4 it will work with RHEL5 but you may have to figure out how to make it do so.

A final note: If you're using RHEL it suggests you want "support" for whatever it is you're doing. If the application vendor doesn't "certify" their product for RHEL5 but does for RHEL4 you may run into a problem when you call for "support". Most vendors will work a problem to a certain degree but when it gets thorny they'll fall back to the "not supported" mantra as a "get out of jail free" card to keep from having to spend more time on it.

Due to this it is important not only to find out if it "works" but if it is "supported".