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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?

Cheers, Rob.

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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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