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Is it important to have all the ram sticks with an equal memory size?

I was told that upgrading our server from 64GB (8 x 8GB) to 72GB (7x 8GB + 1x 16GB) will not yield any performance gain, since the ram sticks will not all be the same size.

Is this true? If yes, can you please explain why?

Server specs: Dell Power Edge Intel Nehalem Dual Socket Quad Core 2.26 GHz. running on Debian. (Is there is a difference running a different OS such as windows?)

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1 Answer 1

up vote 7 down vote accepted

There is a tradeoff. You need to decide if it's worth it. What you might be giving up (depending on your existing configuration) is dual or triple channel mode for your RAM. Effectively, it will make all of your RAM operate more slowly. In most cases, having more RAM will outweigh having faster RAM, but there comes of a point of diminishing returns where the speed of the RAM starts to play a larger factor.

It's hard to give actual numbers on where the trade-off point is because there are so many factors: the base (single channel) speed of your RAM relative to the speed of your disk subsystem, the amount of each and how much RAM you are adding, how much RAM your application actually wants to use, how the cache hit/miss rate changes as your approach that last value, how good your memory controller is, etc.

All else being equal, faster RAM is of course better than slower RAM, and you should strive for matched sticks where possible. But in general, your first job is to get enough RAM in there that, once loaded, your application only rarely needs to hit the disk for page fault reasons. Worry about dual or triple channel options after that.

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3  
+1 and that point comes at the, er, point where you've added more RAM than the system/application needs. It's better (though, I admit, difficult) to have the exact right amount of very fast/matched RAM than lots of slow or unmatched RAM. –  RobM Mar 25 '11 at 18:22
    
Will upgrading a single stick to 16GB affect the speed of all 8 sticks, or will it only affect 2 or 4 of them? –  The Scrum Meister Mar 25 '11 at 18:26
1  
@The Scrum Meister - I think it depends on the system. If you have a multi-socket server with independent memory controllers per socket, it may only effect that socket. But it's more likely it will effect all of your RAM. –  Joel Coel Mar 25 '11 at 18:32
    
@The Scrum Meister: In your case each socket has at least one Memory Controller so max 4 those 4 cores are affected –  mbx Mar 25 '11 at 18:36

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