HP D2200sb Serial Attached SCSI technology, 3rd edition - Page 4

Initiators SAS initiators have multiple ports for connecting to internal and/or external targets. Each initiator port can have a single physical link (a narrow port) or two, four, or eight physical links (a wide port). You can connect SAS initiator ports to separate domains for fail-over redundancy. To find out more about fail-over redundancy, see the "Redundancy in enterprise storage networks using dual-domain SAS configurations" technology brief listed in the "For more information" section. Expanders Expanders connect initiators, targets, and other expanders. They receive commands and data in one port and route them to another port based on the WWN (SAS address) of the target. Expanders use three routing methods-direct, table, and subtractive. Direct routing forwards the commands and data to targets directly attached to the expander. Table routing forwards the commands and data to another expander. When an expander receives an address that it does not recognize, it uses subtractive routing to forward the commands and data to another expander that does recognize the address. Targets SAS disk drives (enterprise-class and midline devices) have two narrow ports. SAS disk drives use the same electrical and physical connection interface as SATA drives. However, SATA drives, including solid-state drives, have a single narrow port. As shown in Figure 1, you can have SAS and SATA devices in a single domain. The size of the expanders' routing tables will determine how many initiators and targets you can have in a domain. SAS drive performance Enterprise-class SAS drives must provide maximum performance 24x7 and under continuous I/O workload in high-vibration environments. Seek time, the time from the initiation of a read or write action until the data transfer, is an important factor in drive performance. The smaller platters of small form factor (SFF) SAS drives inherently yield lower seek times, which is an advantage in file servers with frequent random accesses. HP uses SAS technology to build highly scalable and reliable storage solutions. In enterprise server environments, SFF SAS drives excel in performance and reliability. SFF drives require only 70% of the space and 50% of the power of 3.5inch large form factor (LFF) SAS drives. SFF drives support higher drive densities per U without a significant increase in power consumption. At the same time, higher drive densities give you better overall performance, greater reliability, and lower operating costs. Differential signaling Every SAS device includes one or more transceiver mechanisms, PHYs, in its ports. A physical link of two wire pairs connects the transmitter of each PHY in one device's port to the receiver of a PHY in another device's port (Figure 2). The SAS interface lets vendors combine multiple physical links to create 2x, 3x, 4x, or 8x connections per port for scalable bandwidth. 4