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Author: Susan Bobholz, Storage Market Development Manager,
Intel

Does the storage industry really need another storage fabric? There have been heated debates over the use of Fibre Channel, iSCSI, even InfiniBand, as THE best fabric, that continues even today. Yet, as larger and larger SAS-based topologies are being implemented, there is discussion over using Serial Attached SCSI (SAS) as a fabric technology. To fully understand this phenomenon, it's important to understand the roots of SAS and how SAS systems are being architected.

SAS was developed as the natural evolution of parallel SCSI, enabling point-to-point drive connections via a serial interface. In its simplest form, one SAS port on a SAS Host Bus Adapter (HBA) connects to one drive. Because SAS drives can be dual-ported, a single SAS drive can connect to two SAS HBAs. This enables redundant systems to be built, such that if one SAS HBA failed, the data on the drive is still accessible via the second HBA.

This is an excellent model for storage inside a server, however in many instances, more storage is needed than will fit into the server chassis. To support direct-attached storage outside of the server, the concept of an expander was defined. SAS Expanders enable a simple switching topology. SAS expanders are often incorporated into SAS Just a Bunch of Disks (JBOD) boxes, enabling a single SAS connection from the server SAS HBA directly to the SAS JBOD. Additionally, SAS expanders allow multiple servers to connect to the same SAS JBOD, as shown in Figure 1. The SAS JBOD can then be shared between multiple servers.

By Paul Vogt, Director of Product Marketing,
Adaptec

The flexibility of Serial Attached SCSI (SAS) allows it to be used as a universal connectivity standard for a wide range of business needs. In addition to its role as a replacement for parallel SCSI, SAS now offers the ability to reduce the cost of servers and storage networks, to create tiered solutions that weren't possible before, and to provide investment protection for SATA installations. While SAS performance and scalability generate a majority of the media attention, it could be the flexibility that SAS brings to storage subsystems that creates the most value for your business.

SAS as a parallel SCSI replacement
SAS maintains support for the proven SCSI command set while offering much better performance, scalability, and availability. Stability and reliability have made parallel SCSI the standard in storage connectivity for twenty five years. SAS offers compatibility with this command set while overcoming the physical limitations of parallel SCSI. Now with SAS, it is possible to take advantage of dual-port drives, redundant connections, failover support and scalability up to 128 attached devices and over 16,000 addressable devices. Performance at 3Gb/s per port can be aggregated into wide-port connection bandwidth.

Author: Rachelle Trent, SAS Product Manager,
PMC-Sierra

As enterprise storage requirements rapidly increase, expanding data throughput and providing scalable storage capacity is crucial. The goal of Serial Attached SCSI (SAS) was to overcome the scalability and reliability limitations of parallel SCSI and to significantly improve the ability to add hard disk drives to a system. To overcome the scalability limitation, SAS uses devices such as expanders and multiplexers (muxes).

Expanders for scalability
SAS expanders provide scalability and enable fault tolerant systems by providing multiple connections between hard disk drives and initiator devices. These redundant links between hosts and drives maintain a connection in case of a system failure. In addition, SAS expanders provide high throughput and facilitate the compatibility between SAS hosts and Serial ATA (SATA) disk drives. Expanders simplify the configuration of large external storage arrays and can be cascaded with minimal latency while preserving bandwidth for increased workloads.

Author: Rachelle Trent, SAS Product Manager,
PMC-Sierra

SAS Provides Scalability

• Capacity: maximum 16K devices
• Expanders enable capacity scalability
• Bandwidth: in multiples of 3Gb/s
• Wide links provide bandwidth aggregation
• Increased storage capacity via cascaded JBODs

Author: Tonya Comer, Product Marketing Manager
Industry Standard Servers, HP

Serial Attached SCSI (SAS) is a point-to-point disk interface that builds on the functionality of SCSI for the enterprise. Borrowing reliability and performance enhancements from Fibre Channel, SAS supports dual-port and full-duplex communication between servers and disk drives. SAS extends reliability and bandwidth opportunities by using expanders to create wide ports, which are multi-path connections between the server and storage solutions. SAS also improves manageability of the storage solution by providing the Serial Management Protocol (SMP) to monitor connections between individual devices and identify points of failure or reduced performance.

The first generation of SAS achieves bandwidth as high as 300 MB/s per link and operates in full-duplex mode, in which data, commands, or status information flow bi-directionally. The SAS interface allows for combining multiple links to create 1x, 2x, 3x, or 4x connections for scalable bandwidth. In contrast, Ultra320 SCSI has a half-duplex bandwidth of 320 MB/s per channel.

Author: Tonya Comer, Product Marketing Manager, Industry Standard Servers,
HP

IT managers have traditionally had scant flexibility when choosing a data center storage solution. Their options were limited to separate systems based on parallel Advanced Technology Attachment (ATA), Small Computer Systems Interface (SCSI) or Fibre Channel disk interfaces. These storage options constrained the IT manager's ability to deploy and redeploy core technologies, thereby often adding a significant cost and management burden to the overall enterprise. With the advent of serial I/O architectures, key solution providers, such as HP, can now provide one system or storage solution that will meet the requirements for a broad range of storage applications. This will enable IT managers to standardize on a single server or storage platform, dramatically reducing the complexities of managing storage applications in today's data center.

HP's Serial Attached SCSI (SAS) and Serial ATA (SATA) solutions are the next generation in storage technology, offering better performance, flexibility and scalability. HP has been instrumental in driving the direction of SAS and SATA technologies within the industry. HP designs solutions that are the most advanced and most thoroughly tested, in order to provide the best customer experience possible. HP has ensured that key contributors are unified in the industry direction and development of this monumental technology.

The direction that HP is taking this technology is based on feedback from users—overwhelmingly, the customer has requested a transition from the existing SCSI platforms to the small form factor (SFF) SAS platform. Product development addresses customer pains relating to heat management of newer generation processors by creating drives that draw half the power of their predecessors. It also addresses concerns around physical space constraints, as well as general IT issues such as performance, metrics and density.

Author: Martin Czekalski, Maxtor Corporation

Server blades first appeared about two years ago as a way to deliver increased server compute capabilities within a small footprint. Server blades are offered in a wide range of performance and packaging options. High-density blades (Figure 1) offer modest performance with a focus on low power and high packaging density to maximize the number of servers per rack. This high-density form factor has typically used mobile drives for low power. High performance blades (Figure 2) are focused on providing the maximum compute power in a given space, hence they typically use higher performance enterprise drives (10K and 15K) in a 3.5" form factor. Both blade types utilize a direct-attached storage model for these embedded drives. Additional storage is typically provided by some form of fabric-attached storage (NAS or SAN).

Author: David So, LSI Logic

Video on demand. Tivo. On-line gaming. Digital photography and videography. Video and audio editing. E-mail archives. New SEC and federal regulations (SEC 17a-4, Sarbanes-Oxley, HIPAA, etc.). The increasing need for storage capacity continues down a seemingly endless path, and the timing for Serial Attached SCSI (SAS) couldn't have been scripted any better. SAS introduces greater storage scalability and the ability to access that storage faster than ever before.

SCSI is Good
Today's dominant disk drive interface in the enterprise server and direct-attach storage markets is Ultra320 SCSI, a parallel SCSI speed bump that is nearing its end in innovation. As has been announced by the SCSI Trade Association, the next evolution of SCSI technology beyond Ultra320 is SAS. While Ultra320 SCSI offers many benefits over other currently available I/O interfaces and has served the market well, its physical limitations leave something to be desired as data centers clamor for even greater capacity and higher bandwidth. Parallel SCSI is limited to 15 target devices per bus, and a shared bus at that. With today's prevalent dual-channel SCSI host bus adapters, that's a total of 30 disk drives per HBA.

Authors: Franco Castaldini and Kent Bransford
Seagate Technology

The arrival of Serial Attached SCSI (SAS) marks a new era in storage scalability, wherein both the type and quantity of drives can easily be optimized. SAS compatibility with Serial ATA (SATA) enables seamless deployment of desktop-class SATA drives and enterprise-class SAS drives in the same SAS domain, giving IT managers unprecedented flexibility to specify the most appropriate drive for both online (transactional, high availability) and near-line (archival, low availability) duties. Moreover, employing a common SAS infrastructure for both types of drives minimizes hardware redundancy, further enhancing efficiency.