Expanders: an indispensable component of the SAS architecture - Connectivity

Computer Technology Review, Nov, 2003 by Harry Mason

As the SCSI world began to embrace the successful market migration to Low-Voltage Differential (LVD) SCSI signaling, SCSI expanders emerged as a new class of SCSI devices. The market decided that the robust, yet costly High-Voltage Differential (HVD) or the limited distance, low reliability connections offered by Single Ended (SE) signaling were simply inadequate to serve the SCSI market moving forward. SCSI expanders were introduced as a way to bridge these various signaling domains.

Originally, these devices were created to ease the migration from the legacy SCSI installations to the significantly improved LVD physical connection schemes. These expander devices allowed for logical SCSI domains to be comprised of two or more physical SCSI segments, each with its own signaling capability (HVD, SE or LVD).

These early expanders were often called converters, because they converted the downstream SCSI signaling scheme and were simply modeled in the system as a slight cable delay. It was quickly realized that while SCSI expanders did a great job of converting from one signaling domain to another, they also had the impact of greatly improving SCSI signal integrity by repeating the signal and reducing the impact of the variable bus loading, inherent with the distributed transmission line nature of parallel SCSI. Even in situations where signal conversions were not required, expanders became an excellent way of reducing the interaction between internal and external SCSI buses.

As RAID and clustering became prevalent in the mainstream server market, expanders steadily increased in system usage. Their ability to effectively extend cabling distances, increase the number of devices attached to each parallel SCSI bus and to isolate failed segments of clustered systems (which were often used to provide failover routing in redundant storage configurations), made expanders an essential component in a large number of installations (see Figure 1).

SCSI Goes Serial

Serial Attached SCSI (SAS) is a natural evolution of SCSI beyond Ultra320 SCSI. It is inherently multi-initiator, supports full-duplex transfers, and features dual-ported capabilities--functionality that is not found in desktop-class drives. SAS is designed exclusively as a point-to-point serial interface and provides improved addressing to remove the imposed physical addressing limits of parallel SCSI. Moving to this new connection scheme also reduces the size of the physical connection, making it more appropriate for smaller form factor drives. SAS provides all of these new capabilities, while preserving compatibility with over 20 years of legacy SCSI software.

One of the most significant attributes of Serial Attached SCSI is that it allows SCSI enterprise class drives to coexist with lower cost/gigabyte Serial ATA drives. This important capability of SAS provides an unprecedented level of customer choice and will impact how systems are provisioned to serve different segments of the storage market.

To support this co-existence of Serial ATA and SAS drives, SAS defines three transport protocols:

* Serial SCSI Protocol (SSP): A mapping of SCSI supporting multiple initiators and targets

* Serial ATA Tunneled Protocol (STP): The method by which a SAS host makes a connection to a Serial ATA drive, when using a SAS expander

* Serial Management Protocol (SMP): A management protocol

SAS Expanders

Many of the capabilities of Serial Attached SCSI described above are enabled or enhanced by a new generation of SCSI expanders, called SAS expanders. These SAS expanders are an indispensable component of the SAS architecture. While the point-to-point topology native to the SAS architecture changes some aspects of the traditional SCSI expander, they perform similarly in term of providing the capability to incrementally expand upon the base architecture, while preserving the cost and software advantages of the traditional direct attached storage model.

SAS ports that are physically a part of a SAS host controller may directly address either SAS or SATA drives. The number of drives addressed would be limited by the number of physical ports integrated into the host SAS controller itself, if there were no mechanism of readily expanding the architecture. Integrating a large number of SAS ports into one device could be costly for systems not requiring all the ports in the base controller, yet not providing enough SAS ports would dramatically limit the utility of certain systems. Serial Attached SCSI deploys SAS expanders as the method used for extending the device addressing to the complete range (16K) of devices specified in the SAS standard. It also uses expanders to incrementally expand in-box and near-box storage capabilities in systems requiring greater bandwidth connections, as additional expanders provide redundancy and also provide for addressing large numbers of devices.

[FIGURE 1 OMITTED]

Expanders, in general, offer the ability for a host port to establish an operating connection with the desired device, be it a SAS device, a SATA device or another expander. In order to accomplish these connections, all expanders support an addressing mechanism for routing requests, a means for managing the connections between devices or other expanders and the ability to broadcast primitives across all of the expanded connections it supports.