How to find the latency rating of ram? & Help choosing Server Ram

peacesong

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Jan 7, 2014
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How to find the latency rating of ram?

Trying to determine the best value for upgrading RAM on my new Server and I see hundreds of forum discussions about the importance of latency, posting numbers like:

9-9-9-24-2N
10-10-10-27
5-5-5-15
7-7-7
8-8-8-21
9-9-9-24

But, when I look at lots of ads for buying ram none of them mention any numbers that look like this. How does one find or calculate a 3 to 5 digit latency figure from listings like:

1600MHz DDR3L PC3-12800 ECC Reg CL11 DIMM SR x4 1.35V

Also, most ram listings don’t include the voltage, which is apparently also important.

Then some confusion on UDIMM vs DIMM

More specifically, I just bought a Thinkserver ts140 that came with 1x- 4G of UDIMM and I’d like to upgrade over time, starting with an extra 4G. I’m having a hard time comparing to buy something that matches what came with the machine as there seem to be a dozen flavors of 1600MHz DDR3L PC3-12800 ECC UDIMM out there with the only latency number I see being the CL.

The memory that came with the Thinkserver is: HMT451U7AFR8A-PB which is 1.35v, and to get a 2nd of the exact make would cost $138. But, Kingston ValueRam 4GB is 1.5v (otherwise same specs) for $56 including shipping from Amazon. Would using a 1.35v and 1.5v together be a limiting problem?

UDIMM vs DIMM – most of my research suggests I should be using RDIMM when installing in pairs and claim udimm is less expensive. My price searching is showing udimms being ~30% more expensive.

Lastly, the Ram that came with the server is Single Rank. Some of the upgrade ads mention Dual Rank, but most don't list the ranking. From reading an article at Crucial, this is an important consideration - though they don't list the ranking for their ram upgrade recommendation.

The hardware spec manual for the ThinkServer say it’s good for UDIMMs up to 4G per slot. But, the sales stat sheets show 8G (total of 32G)… so, I’m not trusting the hardware manual and wondering..

Should I throw away the 4G that came with the system and get paired RDIMM? And should I buy all the ram i'll ever need to make sure they're the same latency and voltage since matching these numbers in the future may not be possible?

Many many hours trying to figure this out by reading and will be most grateful for some specific guidance.

Thank you!

Peace
 
The most important of those numbers, the CAS Latency (CL number) is the first number listed, in your case, 11. Dual and single rank memory are simply differences in design architecture of the ram and has very little (some, but not enough to worry about) impact on ram performance. You will want to ensure you get ECC (Error Checking and Correction) ram. RDIMM (registered) will give better performance but I'm inclined to let pricing guide me there but UDIMMs and RDIMMs cannot be used together.
I honestly don't know what to recommend to purchase in this case, all 32GB to load up the system or just 16GB for now, I guess I'll let your budget determine that.