WDTalk

What hardware configurations affect dedicated servers across the board?

Processor speed, cache and CPU Front Side Bus (FSB) are important. Size, speed, type of hard drives and RAID configuration are factors. The amount of RAM is significant as well. But what about the speed of that RAM? How important is it, really?

In comparing identical servers, certainly one with DDR-200 Mhz RAM will underperform today’s standard of 667Mhz DDR2 RAM. Slower memory speeds relate to diminished server performance. Be cautious when spec’ing out your server – even though the FSB may be 1333Mhz, the speed of the RAM may be significantly less. If the RAM speed isn’t listed in a prospective providers offer, simply ask.

When is RAM speed necessarily relevant?

You’ll primarily see a difference in very memory intensive operations. For most, the difference in speed won’t be noticeable. RAM speed accounts for only a few percentage points difference one way or another. In general, speeds limited due to RAM are more likely because you don’t have sufficient RAM, and not because the RAM is too slow.

What happens when your server pages out to disk?

Quite simply, memory runs in nanoseconds and hard drives in milliseconds – no comparison. Once your server starts paging out to disk, server performance will slow dramatically. Consequently, if you’re seeing speeds limited due to RAM it’s more likely you don’t have enough RAM, not because the RAM is too slow. Unless you’re a gamer or benchmark analyst, you probably won’t see much difference between 800Mhz or 1333Mhz RAM.

RAM Explained

ECC (Error Correction Code) versus non-ECC RAM 

ECC RAM will typically run a couple of percentage points slower in performance, but its advantage is increased error handling capability and system resilience. ECC will recover from single bit errors having additional ‘bits’ in order to detect and then correct any ‘parity’ errors in memory. In designing and building servers, select ECC RAM for improved stability.

Peak RAM Speed 

The number (400/533/667/800/1066) represents the raw peak transfer capacity of the memory chips used on the module. Higher speeds have the potential to move data faster, but often that’s accompanied by higher latencies. Frequent small memory access may be better served by lower latency RAM. Frequent large block transfers may be better served by higher peak transfer speeds (despite an increase in latency). 

Unbuffered versus Registered and Fully Buffered 

Unbuffered RAM will generally offer a slight performance boost over Registered or Fully Buffered. Registered and FB adds electrical buffering to the pathway which allows for more RAM to be added to a system but at the cost of increased latency. 

Registered RAM

Registered RAM has buffer chips that increase the amount of RAM a computer can have. You can never use registered memory in motherboards that require unbuffered memory or vice versa.

Both are generally found in server-type hardware, but unbuffered ECC works in desktop/gaming hardware.

Fully Buffered RAM

There’s also “fully buffered” RAM (“FBDIMM” or “FB-DIMM”) which is similar to registered RAM, but unlike normal registered RAM which only buffers the control and address lines, FBDIMMs also buffer the data lines. Similar to ECC (Error Checking and Correcting) memory, FB-DIMM modules perform error checking and correction as well as cyclic redundancy checks on data passing through them, thanks to their AMB chip. The AMBs in each memory channel co-ordinate their error checking efforts so that if a correction or retry is necessary, all the modules stay in sync.

Capacity is where FB-DIMM really does seem to have a clear-cut advantage over conventional memory. Where previously there was a hard 4GB memory limit, modern 64-bit processors can potentially address effectively unlimited amounts of memory at full speed.

Here is a breakup of what you need where:
- AMD socket 754: unbuffered DDR, ECC or non-ECC
- AMD socket 939: unbuffered DDR, ECC or non-ECC
- AMD socket 940: registered DDR, ECC
- AMD socket AM2: unbuffered DDR2, ECC or non-ECC
- AMD socket F: registered DDR2, ECC
- Intel socket 478: unbuffered DDR, ECC or non-ECC
- Intel socket 775 (Core2Duo etc.): unbuffered DDR2, ECC or non-ECC
- Intel Xeon Woodcrest (51×0) and Dempsey (50×0) socket 771: FBDIMM DDR2, ECC
- Previous generation Xeon socket 603/604: registered, can be DDR or DDR2 (depends on chipset/board), ECC
 
With AMD versus Intel CPUs 

In the AMD world, the CPU socket type determines whether you need registered or unbuffered RAM. In the Intel world, it is the chipset and hence the motherboard. That’s why some socket 603/604 boards require registered and some require unbuffered. Existing socket 771 chipsets all require FBDIMM.

Memory Interleaving 

This will typically increase overall performance a couple of percentage points. Interleaved memory allows the system to fetch concurrent banks of data from multiple DIMMs simultaneously. This helps to compensate for the latencies of main memory.