STA

Author: Sam Sawyer
Hitachi GST

The new serial interfaces, Serial Attached SCSI (SAS) and Serial ATA (SATA), have been designed to replace their parallel predecessors. They will support faster data transfer rates and more devices per controller, as well as reduce the size and complexity of the cables and connectors (thus enabling smaller, more densely-packed disk arrays). System builders will also be able to integrate SAS and SATA hard drives in a single enclosure.

SAS goes beyond SATA by adding dual porting, full duplex and device addressing; furthermore, it offers higher reliability, performance and data availability services, as well as logical SCSI compatibility. SAS will continue to enhance these and other metrics as the specification evolves, including increased device support and longer cabling distances.

Author: Paul Griffith
Adaptec, Inc.

Why Choose SAS?
The topmost factor in choosing a storage subsystem for enterprise system development is reliability. Maintaining user access to valuable data reduces total IT costs and increases user productivity. Serial Attached SCSI, a serial bus architecture, has emerged to deliver higher levels of reliability than parallel SCSI for mission-critical, transactional applications that must be online around the clock with no data loss.

To ensure continuous data access when a disk drive fails, multiple initiators have long been used in enterprise computing to provide disk drive access to multiple hosts and host bus adapters or both - an approach that doesn't work well in parallel technology configurations because doing so produces single points of failure that can block access to a device and ultimately to critical data.

A developer benefits from utilizing serial bus architectures that overcome this shortcoming in reliability by supporting a network of dedicated point-to-point device connections and eliminating the single point of failure. The connections also provide full bandwidth to each storage device to boost system performance. By contrast, multi-drop parallel bus architectures share total bandwidth among devices.

Author: Harry Mason
LSI Logic

An often over-looked aspect of system reliability is software quality. Software quality improves as the software is used in real world applications and as it undergoes revisions, until the failure rates are quite low. In short, software run time is a necessary requirement to deliver highly reliable enterprise-class storage solutions.

Serial Attached SCSI (SAS) was architected from the beginning to leverage more than two decades of legacy SCSI software. As such, the middleware applications that have supported SCSI over this lengthy period of time can be reused without sacrificing any enterprise-proven functionality.

Author: Martin Czekalski
Maxtor Corporation

Reliability, availability, and scalability are the cornerstones of online transaction processing (OLTP) and enterprise class computing systems and storage subsystems. In this issue of Serial Storage Wire we will cover the first two as they apply to Serial Attached SCSI (SAS) and cover scalability in the next issue.

Reliability
The terms reliability and availability are often misunderstood and used interchangeably, when in fact they refer to two different attributes of a system or its components. For example, a component's reliability is typically specified as Mean Time Between Failure (MTBF), which represents the mean number of component hours accumulated between failure events when a large sample of components is run in operation. Another important aspect of reliability is the ability to detect errors when they occur and take appropriate action so as not to adversely affect the integrity of a system or its data (error detection and containment).

Availability
Availability, on the other hand, is the percentage of time a system or component is available for use. To calculate availability, additional factors beyond MTBF need to be factored in such as Mean Time to Repair (MTTR) and the overall system architecture. In many cases individual system components can fail while not affecting the availability of a system (e.g. RAID systems can usually remain available, even if a single disk drive fails).