Random Access Memory
Memory selection for servers is a
constantly evolving process. Although naming is very similar between types, the
performance and capabilities differ significantly. Common server memory
implemented today includes double-data-rate (DDR), synchronous dynamic
random-access memory (SDRAM), double-data-rate two (DDR2), and double-data-rate
three (DDR3). Proper memory selection not only is critical to the performance of
the server but also directly impacts the server's ability to stage, alter, and
distribute data throughout the bus efficiently.
Although not as common as other types of
memory, the DDR SDRAM memory type still exists in servers today. This, the
slowest of the DDR RAM types, is considered legacy memory by today's standards.
As a foundation for understanding how DDR memory is rated, consider DDR-266
memory as an example. The first "D" implies that the clock rate of the memory is
doubled. For a server that operates with a 133-MHz front side bus, DDR chips
will operate at 266 MHz. Taking this example to the next step, PC-2100
DDR-SDRAM, a popular-speed memory for early Intel Xeon servers, requires the
same 133-MHz bus, leveraging multiple DDR memory chips rated at 266 MHz. PC-2100
DDR-SDRAM has an overall throughput rating of approximately 2.1 GBps.
DDR2 SDRAM utilizes nearly the same
calculation process as DDR memory, with the exception that the 2 implies that
the original DDR clock speed is doubled. Using the same example as with DDR-266
SDRAM, DDR2-533 SDRAM would be 133-MHz memory chips, clocked at 266 MHz, then
doubled again, to 533 MHz. DDR2-533 chips are based on the same 133-MHz bus
speed as DDR-266. Following through on the example, DDR2-533 chips would be
required for PC2-4200 SDRAM, with an approximate throughput of 4.2 GBps.
DDR3 SDRAM is expected to be the next
generation of SDRAM. The 3 in DDR3 implies that the clock rate and overall
throughput of the SDRAM has been doubled, not tripled. In the example of the
133-MHz bus, the calculations would be as follows: 133-MHz memory would be
double-clocked to 533 MHz and then doubled to 1066 MHz. DDR3-1066 would be the
rating of the chips, with an approximate throughput of 1.06 GBps per chip.
DDR3-1066 chips would be applied toward the PC3-8500 SDRAM, allowing for an
ultimate throughput of approximately 8.5 GBps.
Table 2-3 compares the speed differences and throughput ratings of a
broad array of memory options available today, and into the near future.
Table 2-3. SDRAM Comparison
| Bus Speed |
DDR |
DDR2 |
DDR3 (*future) |
| 100 MHz |
Chip: DDR-200
Module: PC-1600 Throughput: 1.6 Gbps |
Chip: DDR2-400
Module: PC2-3200 Throughout: 3.2 Gbps |
Chip: DDR3-800
Module: PC3-6400 Throughput: 6.4 Gbps |
| 133 MHz |
Chip: DDR-266
Module: PC-2100 Throughput: 2.1 Gbps |
Chip: DDR2-533
Module: PC2-4200 Throughput: 4.2 Gbps |
Chip: DDR-1066
Module: PC3-8500 Throughput: 8.5 Gbps |
| 166 MHz |
Chip: DDR-333
Module: PC-2700 Throughput: 2.7 Gbps |
Chip: DDR2-667
Module: PC2-5300 Throughput: 5.3 Gbps |
— |
| 200 MHz |
Chip: DDR-400
Module: PC-3200 Throughput: 3.2 Gbps |
Chip: DDR2-800
Module: PC2-6400 Throughput: 6.4 Gbps |
— |
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