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The Dell PowerEdge 1950 has eight memory slots. Previously, it had 4 x 2GB in there. We need a bit more memory so added an extra 2 x 2GB. On boot, the BIOS warned us that the preferred configuration is to fill all 4 additional slots but allowed the system to boot.

I'm aware of the requirement for pairs of memory but what have we "lost" by leaving it potentially unbalanced like this?

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The Intel 5000X chipset of your PowerEdge had to disable quad-channel access to memory so you'll have somewhat less memory bandwidth. I don't actually know if it's able to go quad on your 4x2GB and go slower on the remaining 2x2GB but I suspect it can't use different access methods and it'll use the slower mode on the whole memory range.

On many server chipsets softwares like memtest86+ will be able to detect and report the exact memory access mode and effective speed - you could probably use that to compare how the memory subsystem performs with and without the additional 2x2GB.

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Thanks for this, wasn't aware of quad-channel RAM! – Rob Nicholson Sep 30 '10 at 15:27
When checking with memtest or similar, do remember that it is not intended as a real-world benchmark at all. Any speed difference it shows may be much smaller than you'll notice in real-world operation due to the many factors it won't take into consideration (its job being to check for errors, not performance). I suspect the difference will be small anyway - several orders of magnitude smaller than the difference due to reduced disk I/O from needing to swap less and/or from being able to cache more. – David Spillett Oct 2 '10 at 10:08
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Performance - that's it.

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Game of swings and roundabouts then ;-) We've increase memory thus reduced page file thrashing but lost a bit of speed at the same time. 2 x 2GB wasn't horrendously expensive so will probably buy a couple more – Rob Nicholson Sep 30 '10 at 15:30
Oh you'll need the memory more than the slow down - so it's still a win, just not as big as if you'd balanced them. – Chopper3 Sep 30 '10 at 16:25
Make sure you get the exact same spec if you buy more and want a chance of the chipset using them in quad-channel mode. You may need to get a new box of four that is rated for quad use. Memory controllers will err on the side of caution an not enable multi-channel modes if there is any hint of possible timing discrepancies between the modules in a given bank of slots. – David Spillett Oct 2 '10 at 10:02

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