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We're looking to upgrade our existing 2GB x 2 RAM by adding 4x4GB RAM. Will the server encounter some problem or inefficiency by having 2 slots with 2GB and 4 slots with 4GB? Is it recommended all slots be of the same capacity?

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It's a good thing Dell publishes Specs so we can definitively answer questions like this:

Memory: Up to 32GB (8 FBD DIMM slots): 256MB/512MB/1GB/2GB/4GB Fully Buffered DIMMs (FBD) in matched pairs, 533MHz or 667MHz

Ref: Dell Poweredge 2950 Specs

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And that's why I voted for Chris S in the moderator poll :) – thinice Jan 25 '12 at 15:25
It was the avatar change that decided for me :) – Bryan Jan 25 '12 at 16:06

You can mix DIMMs but the machine will throw an annoying message every time you boot up stating memory is not installed in an optimal configuration. You'll miss out on dual channel DDR or whatever it is. Works but slightly slower. We tend to remove old DIMMs when upgrading and install in either 2,4 or 8 dimm configurations and tbh the difference between 16 and 20Gb is not going to be much.

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Yep - the manual says dual channel, but the above is what will happen. – Robin Gill Jan 25 '12 at 16:06

Not for a 2950, I do't think so, as long as you use the same speed RAM across all DIMMS.

If you have the sticks in pairs as a 2950's use DDR2 everything should be fine.

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