With the high data rate of Rambus, signal integrity is troublesome. System boards must be designed to accommodate the extremely stringent timing of Rambus, and this increases product time to market.

Additionally, each Rambus channel is limited to 32 devices, imposing an upper limit on memory capacity supported by a single bus. Use of repeater chips enables use of additional devices and increases potential memory capacity, but repeater chips have been very challenging to design.

Finally, the larger dies and more limited production of RDRAM compared to those of other memory technologies have increased the cost of RDRAM. RDRAM still costs up to twice as much as SDRAM.

RDRAM technology offers performance advantages and lower pin count than SDRAM and DDR SDRAM. However, SDRAM and DDR SDRAM offer more memory capacity and lower cost than

RDRAM.

Importance of using HP-certified memory modules in ProLiant servers

Customers should use only HP memory option kits when replacing or adding memory in ProLiant servers. This section describes two of the most important reasons.

First, not all DIMMs are created equal; they can vary greatly in quality and reliability. In the highly competitive memory market, some third-party memory resellers forego the level of qualification and testing needed for servers because it adds to the price of DIMMs. HP uses proprietary diagnostic tools and specialized server memory diagnostic tests that exceed industry-standards to ensure the highest level of performance and availability for ProLiant servers. The costs of system downtime, data loss, and reduced productivity caused by lower quality memory are far greater than the price difference between HP-certified memory and third-party DIMMs.

Second, use of HP memory option kits prevents improper mixing of single-rank and dual-rank DIMMs. Although single-rank and dual-rank DIMMs may have the same capacity, they differ in the way in which they are accessed by the chipset (see the “DIMM configurations” section). Therefore, to ensure that the server boots properly and operates reliably, single-rank and dual-rank DIMMs should not be used in the same bank. On the other hand, some ProLiant server platforms have configuration guidelines that allow the mixing of single-rank and dual-rank DIMMs. HP memory option kits precisely match the capabilities and requirements of the ProLiant server for which they are designated. Therefore, they prevent improper mixing of single-rank and dual-rank DIMMs.

HP memory option kits are listed in each server’s user guide and in the product QuickSpecs available at www.hp.com.

Conclusion

The increasing performance gap between processors and memory has generated development of several memory technologies. While some memory manufacturers prefer a revolutionary approach to memory technology development, others favor an open, evolutionary approach. Memory manufacturers must balance the cost of performance enhancements against the laws of physics and compatibility with existing technologies. HP will continue to evaluate relevant memory technologies in order to offer customers products with the most reliable, best performing memory at the lowest possible cost.

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