Can a server have "too much" RAM? Is there an optimal level of RAM, or is more always better?

Solution 1:

There are a few thresholds out there for 'too much', though they're special cases.

In 32-bit land, PAE is what allows you to access memory over the 4GB line. The theoretical max for 32-bit machines is 64GB of RAM, which reflects the extra 4 bits PAE gives memory addresses. 64GB is less than 80GB.

From there we get processor-specific issues. 64-bit processors currently use between 40 and 48 bits internally for addressing memory which gives a maximum memory limit of between 1TB and 256TB. Both way more than 80GB.

Unless he has some clear reasons for why SQL Server can't handle that much memory, the base OS and hardware can do so without breaking a sweat.

Solution 2:

He was blowing smoke - if he'd said 4GB and you were using 32-bit operating systems then he might have had half of an argument but no, 80GB is just a number he's pulled out of the air.

Obviously there are some problems if memory isn't 'bought wisely', for instance larger DIMMs usually cost more than twice the price of the half-size versions (i.e. 16GB DIMMS are more that twice 8GB DIMMS) and you can slow a machine down quite a way by not using the right number/size/layout of memory but it'll still be very fast. Also of course the more memory you have the more there is to break but I'm sure you'll be happy with that system for what you're asking of it.

Solution 3:

I apologize but most of these answers are incorrect. There is, in fact, a point at which more RAM will run slower. For HP servers with 18 slots, like the G7, filling all 18 of the slots will cause memory to run at 800 instead of 1333. See here for some specs:

http://www8.hp.com/h20195/v2/GetHTML.aspx?docname=c04286665

(Click on Memory, of course.)

Typical memory config with 12 slots filled will be 48G (all 4s), 72G (8s and 4s), 96G (all 8s), etc... when you say "140G" I assume you really mean 144G, which would very likely be 8G in all 18 slots. This would in fact slow you down.

Now, from what research I have done it appears the slower memory speed doesn't affect a lot of applications, but one thing it is known to affect is database apps. In this case you say it's for a SQL cluster so yes, for that, too much RAM could slow you down.

It's possible the server admin you talked to knew this from practical experience without knowing the exact technical reason.

Hope that helps,

-Jody-

Solution 4:

Take it to an extreme and say you have Petabytes of memory: The system (cpu) is not going to work harder to manage memory mappings. The OS should be smart enough to consume this memory for disk caching, and still have plenty to manage application space (memory and code). Mapping memory in RAM vs virtual space will consume the same amount of CPU cycles.

Ultimately the only thing to suffer with too much memory is wasted energy.