Memory

A file server places large demands on memory bandwidth. This should be taken into account when choosing memory. There are three types of memory that can be used in systems. The type of memory needed will be determined by the motherboard selected. The different memory types will be discussed separately.

SDRAM

This is the slowest of the memory types discussed here. Because of this, every effort should be taken to ensure that adequate memory bandwidth exists. Currently, SDRAM is available with a multitude of options, including: bus speed, registered/non-registered, ECC/non-ECC and CL2/CL3.

The current performance standard for bus speed is 133MHz (PC-133). It is possible to get memory that claims to be able to run at a 150MHz bus speed, but the motherboards that can run their memory bus at 150MHz have been replaced with motherboards that use DDR SDRAM. The memory chips in PC-133 DIMMs should be rated at 7.5ns or faster. Some memory sellers use 8ns memory in PC-133 DIMMs. This should not be used, as it tends to run too hot.

Each motherboard has a specific type of memory (registered/non-registered) that it requires. Use the type of memory that is specified. If the memory type (registered/non-registered) is not specified, then non-registered can usually be used.

The choice between ECC and non-ECC is discussed later in this web page.

Currently, SDRAM memory is available with either CL2 (CAS Latency of 2 clocks) or CL3 (CAS Latency of 3 clocks). Most memory is available with CL3, but, in PC-133, only DIMMs of 256MB or smaller are available with CL2. The CAS Latency is the number of clocks (133MHz in this case) that the Column and Row data must be valid before the memory chips will accept it. I have never seen 512MB PC133 ECC CL2 memory advertised (Corsair Microsystems has a 512MB PC133 non-ECC non-registered CL2 DIMM).

I feel that the file server will run faster with 1GB of CL2 memory than it would with 2GB or 4GB of CL3 memory. This has not been tested.

DDR SDRAM

This type of memory is well suited to use on a file server. It has very high memory bandwidth. Look for CL2. The effect of CL2 vs. CL2.5 is similar to the difference between CL2 and CL3 SDRAM, but less.

RDRAM

This type of memory is even better suited to use on a file server than DDR SDRAM. Unfortunately, it has a high price tag associated with it. My personal opinion is that this is only worthwhile in a data intensive server.

Regardless of what type of memory you choose, if the motherboard will accept ECC memory, it should be used. This will significantly reduce the frequency of data errors from memory, thus increasing the reliability of the server.

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