If you’re used to building desktop PCs, buying DDR3 ECC DIMMs for a server or workstation built around an AMD Opteron is going to take a little learning to be sure you get the right product.
Unbuffered vs. Registered Memory
First of all, you need to determine whether your motherboard takes unbuffered or registered memory. Many server and workstations boards can take both, but some only work with one or the other. Many (perhaps most) desktop PC motherboards only work with unbuffered memory.
What’s the tradeoff between unbuffered and registered memory? Usually unbuffered memory is a little less expensive and a little faster but will not allow as many DIMMs to be used. This is why server and workstation boards favor registered memory as they often have more than the usual two to four DIMM sockets common on desktop motherboards.
Unbuffered memory is generally a little less expensive only for 4GB and smaller DIMMs. For 8GB and 16GB DIMMs, as of late 2011 and early 2012 often registered DIMMs are far less expensive and much more readily available. This may change as 8GB unbuffered DIMMs become more common due to Intel’s low-end E3 server chip line that maxes out at four modules of 8GB each.
DIMM rank is the next thing to check. As it is often used, rank refers to the number of chips “stacked” on the DIMM address bus and/or data bus. Quad rank means there are four chips attached to each pin. This often happens when 8 bit wide DRAMs are used in high capacity memory modules. Dual rank means there are two chips attached to each data bus pin. That is common when 4 bit wide DRAMs are used. Single rank means there is only one, right now you usually only find this with relatively low capacity DIMMs.
The reason that rank matters is that registered DIMMs are not fully buffered so each DRAM chip puts additional electrical load on the memory controller interface. In general, the more the load, the slower the interface can run. That’s especially true when you put more than one DIMM on a DRAM memory controller channel.
Quad rank DIMMs are often the least expensive, but if you install more than one per memory channel you will see the memory bus speed dropped one or more speed levels. For instance, if you install quad rank Kingston 8GB 240-Pin DDR3 SDRAM DDR3 1333 ECC Registered model KVR1333D3Q8R9S/8G DIMMs on an ASUS KGPE-D16 motherboard, they will run at 1333 MHz with one per memory channel. That can get you up to 32GB RAM per processor. But if you want to add a second DIMM per channel, perhaps to expand to 64GB RAM on a processor, then these DIMMs will only run at 1066 MHz instead of 1333 MHz. Memory bandwidth will probably still be comparable because the memory controller will interleave between DIMMs on the same channel.
But if you used single or dual rank DIMMs instead of the quad rank ones in the example above, you would probably see the memory bus still running at 1333 MHz.
It is possible to get dual rank server grade registered DDR3 ECC DIMMs all the way up to 16GB size today. Samsung SDRAM, which is often one of the more expensive brands, is often used on such DIMMs. The 1333 MHz speed grade is most common right now, but 1600 MHz has recently become available.
8GB dual rank DIMMs with chips from Hynix, Elpida, Micron, and Samsung are very easy to find these days. These are probably the cost-effective option for those building a small to medium sized server.
Most DIMMs suitable for use with the Opteron 4100, 4200, 6100, and 6200 series processors are 1.5V. But these processors also support 1.2V and 1.35V memory and so do some of the motherboards that runs these processors.
Usually the lower voltage memory runs a little cooler and uses less power but is initially more expensive. For computers powered on 7/24, lower voltage memory may be advantageous.
There are a few DIMMs that are rated to multiple voltages. Samsung makes DIMMs from its D-series DDR3 DRAM chips that can run at 1.35V and 1.5V so you can use them with a wide variety of motherboards.
Profile or Height
Low profile DIMMs that don’t have large vertical heights from circuit boards or heat spreaders may be needed in some applications because of the overhang from CPU heatsink/fan assemblies.
I’d suggest you buy 8GB DDR3 ECC DIMMs running at 1600 MHz for most new builds using AMD Opterons in the 4200 and 6200 series chips that support 1600 MHz memory. 8GB DIMMs are probably the most cost effective memory versus both 4GB and 16GB DIMMs. Plus they give you a big system RAM capacity range from a single processor dual channel system with 16GB RAM all the way up to quad processor 16 channel systems with 256GB RAM. That covers most of the market pretty well.
If you know you are going to eventually need huge amounts of RAM, then consider the 16GB DIMMs. But be aware as of early 2012 you’ll probably be paying two or more times the price per gigabyte for them, although there are signs now that the price-per-gigabyte penalty may be dropping to around 25% via some new Hynix 16GB 1600 MHz ECC DDR3 dual rank 1.5V DIMMs at SuperBiiz.
Where to Buy
Newegg is often a great place to buy hardware if you don’t have a resale license with relationships with distributors. Unfortunately, they are behind the curve on 1600MHz server DIMMs and still are not carrying any of them months after other vendors have started.
SuperBiiz often has a few 8GB and sometimes also 4GB and 16GB DDR3 1600 MHz DIMMs in stock.
I’ve used the Samsung 8GB DDR3 ECC DIMMs model M393B1K70DH0-CK0 obtained from SuperBiiz successfully with the ASUS KGPE-D16 motherboard and an Opteron 6274 processor. I tried both one per channel and two per channel and in both configurations they ran at the full 1600 MHz speed.
There’s also Ebay as I’ve found the faster DIMMs there, too, but personally am not as comfortable about these being brand-new high quality DIMMs given how a lot of supposedly “new” hardware on Ebay is coming out of server pulls and other sources that might leave you hanging with no return recourse if it doesn’t work at all or for long.