Server RAM

Server Memory

THE PRESENT: RDDR2 (PC2-****R) ECC RAM

Intel says they will not be going to DDR3 for their servers so the following RAM is what everybody should be looking for. (However the hardware market is full of reversals.) The prices are quite reasonable, perhaps too reasonable, so cross check with the memory Intel has verified before buying:

• fb-dimm 1gb ecc 6400 -non
• Pricewatch on registered ECC DDR2 1gb
• Pricewatch on registered ECC DDR2 2gb

Name	PC2-XXXX	Mem clock	Cycle	I/O Bus clock	Data transfers per second	Peak rate
DDR2-400	PC2-3200	100 MHz	10 ns	200 MHz	400 Million	3200 MB/s
DDR2-533	PC2-4300	133 MHz	7.5 ns	266 MHz	533 Million	4266 MB/s
DDR2-665	PC2-5400	166 MHz	6 ns	333 MHz	667 Million	5333 MB/s
DDR2-665	PC2-6000
DDR2-800	PC2-6400	200 MHz	5 ns	400 MHz	800 Million	6400 MB/s
DDR2-900	PC2-7200
DDR2-1000	PC2-8000
DDR2-1066	PC2-8500	266 MHz	3.75 ns	533 MHz	1066 Million	8533 MB/s
DDR2-1150	PC2-9200
DDR2-1200	PC2-9600

For use in PCs, DDR2 SDRAM is supplied in DIMMs with 240 pins and a single locating notch. DIMMs are identified by their peak transfer capacity (often called bandwidth). Note: DDR2-xxx (or DDR-xxx) denotes data transfer rate, and describes raw DDR chips, whereas PC2-xxxx (or PC-xxxx) denotes theoretical bandwidth (though it is often rounded up or down), and is used to describe assembled DIMMs. Bandwidth is calculated by taking transfers per second and multiplying by eight. This is because DDR2 memory modules transfer data on a bus that is 64 data bits wide, and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer.

In addition to bandwidth and capacity variants, modules can

1. Optionally implement ECC, which is an extra data byte lane used for correcting minor errors and detecting major errors for better reliability. Modules with ECC are identified by an additional ECC in their designation. PC2-4200 ECC is a PC2-4200 module with ECC.
2. Be "registered", which improves signal integrity (and hence potentially clock speed and physical slot capacity) by electrically buffering the signals at a cost of an extra clock of increased latency. Those modules are identified by an additional R in their designation, whereas non-registered (a.k.a. "unbuffered") RAM may be identified by an additional U in the designation. PC2-4200R is a registered PC2-4200 module, PC2-4200R ECC is the same module but with additional ECC.

So this is all quite simple and easy to understand, yet if you do a search for "pc2-5400r ecc" you'll find results that turn up stores in other countries. Apparently the rest of this world goes by this spec but Americans list memory in an almost unique way at every store it's sold at.
Note: registered and unbuffered SDRAM generally cannot be mixed on the same channel.

THE FUTURE: RDDR3 (PC3-****R) ECC RAM

Some are pontificating new chip sets will be developed for ECC RDDR3 memory in the first half of 2008. And of course this memory will need to be produced. Apparently the TDP (Thermal Power Design) of RDDR2 is about 8 times higher than RDDR3. Then there's the possibility a cooler chip can pack in much more memory at higher clock speeds. Coupled with the fact AMD does not support RDDR2 for their servers it is a good bet RDDR2 only has a few years of life. It's almost 2008 now so it might not be too surprising to see RDDR3 by 2010 as a standard uniting AMD and Intel. Until then RDDR2 ECC is the obvious and even inexpensive standard we can live with.