Serial Attached SCSI for External Subsystems


Market Timing
Higher Performance
Increased Availability
Utmost Flexibility
Seamless Scalability & Improved Cost of Ownership

Introduction   top of page

Serial Attached SCSI (SAS), the follow-on to parallel SCSI, is designed for high-performance enterprise requirements and offers interoperability with Serial ATA (SATA), bringing enterprises a flexibility and cost savings previously not possible. SAS provides significant benefits to external storage subsystems and offers users "one-stop-shopping" to satisfy their requirements for the following three main data types;

Throughput Data Transaction Data Reference Data
  • High MB/s and large data-intensive files
  • Large block, random read/writes
  • Video streaming
  • CAD
  • Maximum IOPs for OLTP, calculation intensive files
  • Small block, random read/writes
  • Reservation systems
  • Billing systems
  • Fixed content, archival data for secondary/nearline storage
  • Large block, sequential writes/reads
  • Imaging
  • Medical records
Figure 1

An external subsystem with a SAS midplane will support both high performance, reliable SAS drives ideal for transaction data, and cost-effective, high-capacity SATA drives ideal for reference and throughput data. Now customers can use one platform instead of two to support both drive types and satisfy all three application segments (illustrated in Figure 1). SAS-based external storage arrays provide benefits in five areas: performance, availability, flexibility, scalability, and cost of ownership.

Market Timing   top of page

Because of its trusted reliability and stable feature set, the SCSI protocol has been accepted by the market for a remarkable twenty years. Introduced three years ago, SAS leverages this continuing evolution of SCSI with new levels of scalability, flexibility, and cost-effectiveness for connectivity, data transport, and data storage.

The first SAS prototypes were announced in 2003 and were a major step to achieving mass market availability. Those prototypes allowed development of the first generation of technologies and products that will bring the benefits of SAS into the enterprise. These products have now been developed and tested, and a wide variety of integrated solutions have been demonstrated.

Interoperability testing was a key component of SAS, because it increases the architecture's flexibility by supporting both SAS and SATA disk drives and components. Interoperability allows one vendor's SAS products to be compatible with another's, and it also ensures products developed today will work with all existing and next-generation SATA products.

Interoperability testing was conducted throughout 2004, and the University of New Hampshire InterOperability Laboratory successfully demonstrated all the required levels of interoperability in a variety of SAS products and configurations.

The same companies that pioneered SCSI are investing in its evolution to maintain its high reliability standards while meeting the market's evolving technology and functionality requirements. Industry support comes from all of the major disk drive vendors, host adapter suppliers, chipset manufacturers, large computer makers, and many other suppliers.

A wide variety of recent product and solution announcements have been made, with some product currently shipping. Others are expected to ship in early 2005 to make the long promise of SAS a reality. SAS subsystems are expected to ship in the first quarter of calendar 2005, and will coincide with the availability of SAS disk drives and SAS host bus adapters. Volume shipments will commence in the second quarter.

Higher Performance   top of page

SAS has received extensive market attention due to several performance benefits. For example, a SAS connection can support four SAS wide lanes or 4 x 300MB/s per connection.

A SAS connection on a JBOD system can support a theoretical maximum of 1,200 MB/s (see Figure 2). SAS is also used as a high performance, yet cost-effective expansion port to daisy chain to another SAS subsystem.

JBOD System
Figure 2

In addition, each 3Gb high-performance SAS drive (10K rpm or 15K rpm) is connected to a 3Gb drive connection, providing more then enough bandwidth for the highest performing drives (see Figure 2). Assuming 16 SAS drives in a subsystem and each drive capable of 75MB/s, that would equal 1,200 MB/s performance from 16 drives which far surpasses both 2Gb Fibre Channel loops and U320 Parallel SCSI buses. The roadmap for SAS is robust and provides clear investment protection by starting at 3Gb, doubling to 6Gb in 2007 and then doubling again to 12Gb.

Increased Availability   top of page

Another benefit of SAS-based storage subsystems is the native dual port capability of each SAS drive, providing a redundant path to each drive in the event of a adapter failover. In addition, since each drive is on a separate point-to-point connection, losing a drive connection only affects a single drive. Compare this to parallel SCSI in which losing a bus affects all the drives on the bus.

SAS drives, like Fibre Channel, are designed for the rigors of enterprise use and heavy loads, have MTBF ratings in excess of 1 million hours, and warranties up to 5 years.

SAS drives are engineered for rugged enterprise duty, and every component (drive motor, spindle, actuator, firmware, etc.) is specifically designed and manufactured for that rigorous use. SAS drives also safeguard data integrity via their comprehensive verification/error correction capabilities.

SAS drive-based subsystems support active-active adapters whose failover and redundant host connections can be SAS, Fibre Channel or iSCSI. These subsystems also support redundant hot swappable power, cooling, and enclosure services and the same RAID and software capabilities found in Fibre Channel and SCSI-based arrays.

Utmost Flexibility   top of page

Probably the best known benefit of SAS for external storage is its configuration flexibility. A SATA drive can be plugged into a SAS midplane without any modifications or changes, and the adapter in the array and the software seamlessly support a system with a mix of SATA and SAS. As illustrated in Figure 3, 6 SAS drives could be partitioned in one storage pool or LUN and assigned to a server with a transaction-intensive application, such as a reservation system. The other 6 drives could be SATA and partitioned in a storage pool or LUN and assigned to a different server running a reference application, such as medical imaging. Alternatively, one array could support all SAS drives and a second array connected or daisy chained to the first could support all SATA drives. This flexibility is not possible with Fibre Channel and Parallel SCSI-based subsystems; only SAS provides customers with the best of both worlds.

SATA Drive
Figure 3

Seamless Scalability & Improved Cost of Ownership   top of page

Unlike SCSI, whose architecture limits the scalability of multiple drive enclosures, SAS provides the capability to scale more than 100 drives, or 36TB with 300GB SAS drives or 48TB with 400GB SATA drives. This provides very large pools of storage and a cost-effective alternative to Fibre Channel-based SANs.

SAS drives are expected to be priced on par with SCSI drives. SAS implemented as a host interface and expansion interface on a RAID adapter or JBOD I/O module will be less costly then Fibre Channel. In addition, supporting SAS and SATA drive types in one system reduces cost of ownership, simplifies training and product portfolios, and provides investment protection.

Summary   top of page

Serial Attached SCSI will offer new levels of performance, availability, and customer choice by supporting both enterprise-class SAS drives and SATA drives for cost-sensitive applications. OEMs and IT managers will have the flexibility to configure storage subsystems with either drive technology, or both, enabling high-performance and low-cost storage in the same subsystem thus maximizing customers' return on investment and providing flexibility for future growth.

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