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ISS Technology Update, Volume 8, Number 4

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ISS Technology Update Volume 8, Number 4
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Unbuffered DDR-3 DIMMs are a cost-effective solution for ProLiant G6 servers
HP ProLiant G6 servers support Double Data Rate-3 (DDR-3) Synchronous Dynamic Random Access Memory (SDRAM)
technology. DDR-3 SDRAM technology doubles the peak bandwidth of DDR-2 when operating at the same memory bus
frequency. Memory manufacturers are producing two types of DDR-3 SDRAM DIMMs: Registered DIMMs (RDIMMs) and
Unbuffered DIMMs (UDIMMs).
ProLiant G6 servers support single-rank, dual-rank, and quad-rank RDIMM configurations. RDIMMs contain a register that
buffers the address and command signals from the memory controller. The register also reduces the electrical load on the
memory controller, allowing up to 3 DIMMs per channel (DPC), higher capacity per DIMM (2, 4, or 8GB), and faster bus
speeds than UDIMMs. However, the register also increases the power requirements of each RDIMM by almost 1 Watt and adds
1 bus clock of latency. RDIMMs are identified with an R suffix in the module manufacturer’s name (example PC3-8500R).
Select ProLiant G6 servers support both single-rank and dual-rank UDIMM configurations. UDIMMs use less power and cost less
than comparably sized RDIMMs; however, the capacity of UDIMMs is limited to 1GB or 2GB. ProLiant G6 servers that support
UDIMMs, support a maximum of two UDIMMs per channel, which allows a maximum memory capacity of 24 gigabytes using
dual-rank 2-gigabyte DIMMs. UDIMMs are a very cost-effective solution for servers with memory capacity of 24 GB or less.
ProLiant G6 servers ship standard with RDIMMs, which are required to reach the maximum capacity of each server. Select
ProLiant G6 servers support both RDIMMs and UDIMMs; however, RDIMMs and UDIMMs cannot be used together in a system.
Customers should consider using UDIMMs when cost and power consumption are top priorities. HP simplifies memory selection
by providing an on-line ProLiant Memory Configuration Tool (www.hp.com/go/ddr3memory-configurator) to help configure
each server’s memory and provide an orderable parts list.
Table 1. Comparison between UDIMMs and RDIMMs in HP ProLiant G6 servers
Table head
UDIMMs
RDIMMs
Maximum speed supported
DDR3-1333
DDR3-1333*
Ranks per DIMM
1 or 2
1, 2, or 4
Maximum DIMMs per channel
2 dual rank DIMMs
3 dual rank DIMMs
Maximum memory capacity
18-slot servers
12-slot servers
24 GB
24 GB
144 GB
96 GB
*Maximum speed for quad-rank RDIMMs is DDR3-1067
Additional resources
For additional information on the topics discussed in this article, visit:
Resource
Memory technology evolution: an
overview of system memory
technologies, 8
th
Edition
DDR3 Configuration
Recommendations for HP ProLiant
G6 Servers