Transitioning to SAS technology: a comprehensive comparison between SAS and parallel storage

Computer Technology Review, March, 2004 by Tonya Comer, Lorrie Chambers

* Support for multi-port and multi-path connections

* Support for topology and link management

* Support for enclosure management

* Support for the design of new, small form factor hard-disk drives

* Improved backplane support

As a result, vendors can use the same Stock Keeping Unit (SKU) for the backplane to support both SAS and SATA drives. This reduces interoperability issues and inventory costs and simplifies product support costs.

Advantages of Point-to-Point Architecture

SAS, as a point-to-point architecture, establishes a link directly from the controller to a disk drive or through an expander-switching matrix. However, in existing parallel SCSI, only two devices can communicate at once, and as throughput needs increase, the shared-access medium can become a bandwidth bottleneck which affects scalability. Shared access topologies also are typically more complex and have arbitration schemes that are more time consuming than point-to-point architectures.

The ability to configure arrays with low-cost SATA drives or high-performance, dual-port SAS drives will simplify storage purchasing and deployment by enabling one storage array to meet a broad range of application requirements. In addition, SAS utilizes a smaller connector than parallel SCSI disks. The smaller connector is conductive to the design of high-density, small-form-factor servers, hard disk drives, and RAID arrays.

Universal Interconnect

SAS technology makes a significant step toward a universal interconnect for DAS. A SAS initiator supports four standard SCSI protocols including:

* Serial SCSI Protocol (SSP) supports full duplex connections to SAS devices. SSP maximizes throughput and minimizes latency to complement the high-performance characteristics of SAS disk drives. In a heavily queued environment, SSP supports data transfers in both directions simultaneously, so the effective throughput of a single 3-Gb/s link can increase to 6-Gb/s.

* Serial Management Protocol (SMP) supports connections to the SAS expanders within the topology to manage links between devices.

* Serial ATA Tunneling Protocol (STP) supports half duplex connections to the SATA devices attached to expanders. Expanders using STP allow SAS controllers to generate simultaneous requests to multiple SATA devices.

* SATA also supports half duplex connections in SATA devices directly attached to the SAS controller. A SATA device directly attached to a SAS controller effectively forces the SAS controller to become a high-performance SATA controller.

Cabling and Hot Swap

SAS technology continues in the SCSI tradition of allowing hot swappable disk drives. This technology further allows SATA disk drives to be hot swapped, making possible online replacement of the lower cost and lower reliability SATA drives in RAID solutions enabling deployment of SAS with SATA drives in less demanding or cost-sensitive applications. Designed from the onset to provide cost-effective hot swap support for disk drives, SAS has built-in protocol support to ensure SAS drives safely come online and avoid unnecessary loads on system power supplies.