Author: Russ Fellows, Analyst
Evaluator Group
Why Consider SAS?
Serial Attached SCSI (SAS) is both a storage interface to disk and tape drives, as well as a storage connectivity technology. As the natural evolution of the older parallel Small Computer System Interface (SCSI) technology to modern technology, SAS provides application and device driver investment protection. It also provides investment protection by providing system architects another alternative for midrange storage networks while supporting both SAS and SATA storage devices.
In many ways, SAS uses the best of existing disk and connectivity technologies, leveraging Advanced Technology Attachment (ATA), SCSI and Fibre Channel (FC) technology. SAS is a continuation of the highly successful SCSI protocol, updated with current serial-attached connectivity technology. Additionally, SAS connectivity supports both high-performance SAS drives and high-capacity Serial ATA (SATA) drives simultaneously. SAS connectivity, along with SAS and SATA drive choices, provides users with the ability to mix and match drive types to best meet their business application requirements. These factors all serve to provide investment protection with past storage protocols while utilizing current technologies when choosing SAS technologies.
Prior to SAS, the connectivity choices for direct-connected external storage were limited to SCSI, or high-cost FC. SCSI was cumbersome, offered limited connectivity and expansion choices. The move to FC storage was often a large jump in terms of cost and complexity, which in many cases was not worth the investment for small enterprise deployments. The emergence of SAS provides another alternative, positioned as an expandable external connectivity technology, offering enterprise features for both direct and networked connected external storage deployments.
SAS combines the following benefits:
- Leverages the well known and proven SCSI protocol
- Interoperability with SATA drives, offering low-cost high capacity for near-line applications
- Serial technology improves speed, expandability and error recovery versus bus technologies
- Provides full duplex and dual porting for enterprise environments
- Offers another alternative for storage networks with moderate scalability needs without high cost or complexity
As a result, SAS storage technologies are applicable across a broad range of SMB and enterprise environments.
Enterprise Storage Trends
There are a number of I/O connectivity technologies in use today. One of the oldest I/O connection technologies for open-system environments is SCSI. SCSI was both an I/O protocol, as well as a connection technology that emerged in the early 1980’s. The use of the SCSI transport and SCSI protocol were synonymous until the advent of FC.
When FC was developed in the mid 1990’s, it was designed as a transport interconnect, not a storage protocol. By design, FC is able to transport virtually any protocol, including TCP/IP, and a number of other protocols. The most common protocol used with FC is the SCSI-3 protocol, which when transported over FC is known as FCP. Currently, the term “Fibre Channel” has come to be synonymous with FCP (SCSI) transported over a FC interconnect.
Another trend that has evolved is the emergence of serial I/O technologies replacing parallel methods for data transport. This has occurred with SCSI to SAS, HIPPI to FC, IDE/ATA to SATA, RS-232 to USB and FireWire and other transitions. The common aspect in all of these examples is that parallel I/O technologies have been replaced by newer serial techniques for transporting data. There are a number of reasons for this transition. One of the primary factors is greater immunity to data loss, hence fewer distance limitations. Another important aspect is the point-to-point nature of serial connections versus parallel bus technologies. Point-to-point architectures provide the ability to locate and fix failures, as well as eliminate the impact of failed devices on the remaining elements. Still another advantage has been the ability to use smaller connectors and interfaces, which accommodate multiple connections and smaller form factor devices.
With the transition from parallel SCSI I/O (referred to herein simply as SCSI) to SAS, the benefits are numerous. Current parallel SCSI technologies, such as Ultra320 SCSI, require 68 conductors, with cables consisting of 34 wire pairs. SCSI also utilizes a bus architecture, which offers little immunity from link or device failures and requires electrical terminators at the ends of SCSI busses. All of these factors serve to drive up the cost and complexity while lowering the overall reliability of SCSI bus technologies in comparison to the more modern SAS with its point- to-point architecture.
Current Drive Technologies
A disk drive interface is a choice that is independent of the form factor, and independent from the storage networking technology utilized. A disk drive with a SAS interface may be any form factor, utilize either enterprise or desktop class mechanics, and be used with a variety of storage connectivity technologies. There are several types of disk drive technologies in use today. The terms are often interchanged by vendors, industry articles and users, which may confuse the understanding of what pieces of technology are relevant.
With respect to disk drives, there are three aspects that are most relevant:
- I/O interface to the disk (SAS, FC, SCSI, Parallel ATA, (PATA), SATA, etc.)
- Disk drive mechanics (enterprise or consumer/desktop)
- Disk form factor or size (3.5 inch, 2.5 inch, etc.)
An interesting note is that there is nothing inherent in the SCSI, SAS, SATA or FC disk drive standards or specifications that dictate the drives performance or reliability. It has evolved that enterprise users require multiple ports, high availability and high I/O performance. As a result, the two I/O interfaces that allow multiple ports (SAS and FC) are coupled with high performance and reliability drives. However, it is certainly possible for disk drive manufacturers to produce a highly reliable and performing drive and couple it with a SATA interface. It is also possible to combine enterprise connectivity (such as FC) with ATA drive mechanisms which are known both as FATA and low-cost FC drives.
SAS disk drives typically utilize enterprise class mechanics, with mean time between failure (MTBF) characteristics of over 1.2 million hours. Enterprise mechanics are designed for heavy use in 7X24 hour environments. Alternatively, desktop devices, including SATA drives are typically specified for 5X9 hour environments with less than 1 million hours MTBF. Consumer storage devices typically have larger latencies with a correspondingly lower I/O rate than enterprise class disk drives.
Table 1: Enterprise Disk and Tape Connectivity Alternatives
SAS technology is often associated with the next generation of enterprise disk drive form factor, sometimes known as SFF (Small Form Factor) drives. These drives are 2.5″ wide, versus most current enterprise drives in use that are 3.5″ wide. Drive vendors have created some confusion when they released SFF drives only with SAS interfaces. However, as noted, drive form factor is not associated directly with a drive’s interface.
Business Requirements Influence on Disk Storage
There is never one correct answer for technology or deployments, just as there is not one homogeneous set of business requirements. Moreover, customers now have additional choices for network transports and for storage connect technologies.
For some application needs and environments, storage within a server is an appropriate solution. These deployments are the most appropriate for environments with a minimal need for server or storage redundancy. In the hierarchy of deployment scenarios, the next step up in reliability and availability is to move the storage outside of the server, allowing access from multiple systems. Multiple systems may utilize an HBA RAID card to share external storage enclosures, although they would typically not share data in this manner.
Another solution that provides more flexibility is to move the RAID function outside of the server, and place it within the storage enclosure. In this scenario, it is also possible to provide redundant RAID controllers within the external storage enclosure. In a properly architected environment, it is thus possible to have complete redundancy, with no single point of failure. External SAS connectivity supports all three of these scenarios.
SAS controllers can connect to a total of 128 expanders and targets within a single SAS network, known as a domain. Additionally, SAS initiators may have connections to two domains for fail-over and redundancy.
Figure 1: Disk and Storage Networking Deployment Scenarios
SAS connection technology provides a wide range of deployment flexibility. At the low end, SAS connectivity supports small enterprise applications with internal SAS RAID drives. In midrange environments, SAS connectivity supports external SAS and SATA drives. Externally connected iSCSI and FC-connected deployments will also utilize SAS and SATA drives. For large scale enterprise environments, SAS connectivity will typically not meet the high degree of scalability required for these deployments. However, SAS drives are appropriate for these environments, particularly when coupled with FC or iSCSI connectivity.
Moreover, SAS connectivity is appropriate for small-to-midrange deployments, with SAS drives being appropriate across enterprise deployment scenarios.
SAS Connectivity Versus Alternatives
There are a number of advantages that SAS offers over SCSI and other competing transport technology choices. Perhaps the biggest advantage is the interoperability of SAS with SATA. By sharing a physical infrastructure, including the cables, chip sets and connections, it is possible to construct systems that support both SAS and SATA disk drives simultaneously.
There are currently vendors that support intermixing of enterprise disk devices (typically FC drives) with SATA drives. However, these systems must duplicate the entire physical infrastructure, including the connectors, cables and chip sets necessary to support both types of disk devices.
The SAS standard actually supports three protocols for I/O and management of devices. These include the Serial Management Protocol (SMP), the Serial SCSI Protocol (SSP) and the SATA Tunneling Protocol (STP). These are used respectively to manage connections and devices (using SMP) and to transport data for SAS drives (using SSP) and SATA drives (using STP).
SAS is full duplex, allowing data transfers to occur in two directions simultaneously, unlike parallel bus architectures such as SCSI. SAS also provides dual-port connections providing I/O path redundancy critical to enterprise deployments.
Aggregating multiple links is also supported, with so called “multilane SAS” links providing 2, 4 or in some cases 8 SAS channels or ports. These multiple links are often used on HBAs to support either multiple disk drives or to provide high bandwidth to a SAS expander.
Only recently, with the advent of switched disk connectivity, has FC utilized switched connections for disk devices. The majority of legacy FC systems utilized arbitrated loop (FC-AL) which is essentially a point-to-point architecture with a loop physical topology. Each drive must arbitrate, or wait for a period to communicate with the controller, thereby slowing down the connection with each drive added. In contrast, SAS is both a physical and a logical point-to-point architecture, with high drive counts supported by SAS switches. In this way, SAS and modern switched FC JBOD (SBOD) connections are similar.
Table 2: Enterprise External Storage Connectivity Alternatives
SAS Futures
In order to provide a clear path for both vendors producing SAS technology, and consumers that adopt the technology, the SCSI Trade Association has a clear roadmap for SAS. This roadmap includes doubling the interface speed to 6Gb/s in 2008 and then moving to 12Gb/s approximately four years later.
One emerging technology directly related to SAS is the advent of SAS expanders. A SAS expander is essentially a switch for SAS devices, that resides between a SAS initiator, such as an HBA, and an end device, such as a disk or tape drive.
With the combination of SAS expanders and SAS devices, it is possible to create a storage network using SAS interconnection exclusively. This would support the use of both SAS and SATA devices, along with the management of the enclosures and devices via the SAS standard protocols.
The possibility to create another type of storage network provides unique challenges and opportunities for system architects. However, SAS also provides system architects an opportunity to match the most appropriate technologies to a particular deployment.
SAS technology allows architects to deploy storage environments with direct-connect external storage, and later grow the same environment to support multiple host hosts, with multiple enclosures, along with SAS and SATA drives. Thus, architects and administrators have the flexibility to scale from a departmental non-redundant storage environment to a larger environment with redundant systems and paths, all while leveraging SAS interconnections and SAS storage devices.
SAS networks were not designed to replace FC connectivity in large enterprise environments. However, it should be considered in smaller networked storage deployments due to its moderate degree of scalability and ability to construct highly reliable storage deployments supporting both enterprise-class SAS drives and high-capacity low-cost SATA drives where appropriate.
However, SAS disk drives are appropriate in a wide variety of environments ranging from internal storage, to networked external storage up to large scale FC-connected enterprise deployments leveraging SAS and SATA drives.
About the author:
Russ Fellows, analyst, is responsible for leading research/analysis of product and market trends for Network Attached Storage (NAS) and Storage Security. He is also the primary analyst for coverage of selected open systems Storage Area Network (SAN) and virtualization products. Additionally, Russ will participate in developing and maintaining course content for the Evaluator Series Advanced Education for Storage Professionals™ classes. Russ is a well regarded and successful industry professional with over 17 years of high technology experience including product design, product development, systems engineering, business strategy development, competitive analysis and portfolio management within both the vendor and end user communities. His involvement in both communities gives him a unique industry perspective and ability to respond to varied client business and strategy requirements.
