Today I had the opportunity to set up a Dell PowerEdge server. It seems to be a powerful machine - it has 12 RAM slots, 8 SCSI drive bays, 2 CPU slots, 2 Ethernet ports, 2 power inputs, and many noisy fans. (Aside: starting it up tripped the circuit breaker because it drew so much power.) The trickiest part of getting it up and running was getting the RAM arrangement correct.
It is a used server, and one of the RAM modules was bad. Fortunately, the boot process gives all sorts of information on hardware misconfigurations, so finding the bad module was easy. The slots are numbered from 1 to 12, and arranged into four groups of three, with a white slot as the first slot in the group. Rather than moving linearly, the numbering skips from one group to the next before moving to the next slot in each group. So, slot 2 is the first slot (the white one) in the second group, while slot 5 is the second slot in the first group (because slot 4 would be the first slot of the last group).
Then there are pairs of slots. Slot 1 is in a pair with slot 2, #3 with #4, et cetera. Slots in a pair must be filled with the same size and speed of RAM module, so having a module in slot 7 but not in slot 8 will cause the boot process to fail. Having a 1GB module in slot 1 but a 512MB module in slot 2 will also cause a boot fail.
Slots must be filled sequentially or BIOS will complain about some modules being "electrically isolated." Therefore, having slots 1 and 2 empty and 3 and 4 filled will cause a boot fail.
Finally, it is recommended - but not required - that every layer of 3 be either completely full or completely empty. BIOS will warn you if you have slots 1 through 6 full but 7 and up empty because slots 7 and 8 would complete the layer started by 4 and 5. I do not know how bad the performance hit is for such a misconfiguration.
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