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:
| 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
- 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.
- 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.
