We've got two 2950s running RHEL5, each with 4x2GB DIMMs installed. We purchased 4x4GB DIMMs to upgrade the memory in both. We installed two of the new 4GB DIMMS in each server last night, so that each server's DIMM slots were set up as follows::
2GB 2GB 2GB 2GB 4GB 4GB [empty] [empty] We booted, and were greeted with the following warning: "Warning: The current memory configuration is not optimal. Dell recommends a population of 2, 4, or 8 DIMMs. DIMMs should be populated sequentially starting in slot 1." The online manual explains the warning as: "System has detected a legal but non-optimal population of DIMMs (for example, 1 DIMM, 6 DIMMs, 4 DIMMs in slots 1, 2, 5, and 6, etc.). The system will run with all memory accessible but will experience sub-optimal performance." The System Memory section of the manual continues: "General Memory Module Installation Guidelines: To ensure optimal performance of your system, observe the following guidelines when configuring your system memory... FBDs must be installed in pairs of matched memory size, speed, and technology, and the total number of FBDs in the configuration must total two, four, or eight." The machines still ran fine (and faster than before with the new RAM) and both saw all 16GB of total RAM. Of course, the solution to make this warning go away was to simply put 8x2GB DIMMs in one server, and 4x4GB DIMMs in the other. However, it did get me thinking. What EXACTLY does "sub-optimal performance" mean, and what precisely (in as technical terms as possible) was causing it? I didn't feel like searching for the answer at 1:00AM this morning when I got home from the colo facility. But I tried today, and found nothing with Google. I called Dell Tech Support, and they couldn't explain it either. My best guess is that maybe all installed DIMMs will run at the clock speed of the slowest DIMM in the bunch, but again - that's a total guess. I realize this isn't exactly a Linux question, but I figured fellow Linux geeks would be the best group to take a stab an answering a question that neither Google nor Dell Support can. J So... any takers on explaining exactly what the "sub-optimal" performance means in that warning? Am I losing speed, reliability, part of the available RAM, or what? It's an entirely academic question at this point, since moving the DIMMs around made the warning go away, but it's still got me scratching my head. Thanks and Merry Christmas, Steve CEO, CheatCodes.com
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