HP DL360 Serial Attached SCSI storage technology, 2nd Edition - Page 4

SAS domain (Figure 1). Each SAS device has a unique worldwide name (SAS address) assigned at manufacturing to simplify its identification in a domain Initiators SAS initiators have multiple ports for connection to internal and/or external targets. Each initiator port can have a single physical link (a narrow port) or 2, 4, or 8 physical links (a wide port). SAS initiator ports can be connected to separate domains for fail-over redundancy. Expanders Expanders establish connections between initiators, targets, and other expanders by receiving commands and data in one port and routing them to another port based on the SAS address of the target. Expanders use three routing methods-direct, table, and subtractive. An expander uses direct routing to forward commands and data to targets directly attached to the expander. An expander uses table routing to forward 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 hard drives (enterprise-class and midline devices) have two narrow ports. Each port can reside in a different SAS domain to provide fail-over redundancy and load balancing. SAS hard drives leverage a common electrical and physical connection interface with SATA hard drives. However, SATA hard drives, including solid state drives, have a single narrow port. Figure 1. Example of SAS devices in a domain. The number of initiators and targets allowed in a domain is limited only by the size of the expanders' routing tables. 4