Small computer system interface (SCSI) technology emerged in the data center with the advent of Serial Attached SCSI (SAS) and rapidly became the storage interface of choice for servers and external storage products. In order to accomplish this, SAS proponents had to address three fundamental areas:
- The reliability, accessibility, and serviceability required by enterprise storage
- A price/performance point that exceeded other solutions
- The ability to scale beyond traditional directed-attached storage (DAS) models
Meeting enterprise demands
Data centers require reliability to protect and maintain the accuracy of sensitive information such as financial or medical records. To address reliability, SAS provides hardened error handling and specifies bit error rates of less than 10-12 to ensure there is no loss of data.
Accessibility is the measure of operational time of the system relative to the total time, and it can approach 100% by eliminating any single point of failure. Multi-pathing is the technique within the storage subsystem that ensures that no single point of failure will compromise operation. Multi-pathing is native to the SAS protocol and can help data centers achieve the levels of accessibility they need.
Serviceability is the ease with which a component, device, or system can be maintained and repaired. SAS offers numerous diagnostic features (such as error counters) that enable administrators to monitor the integrity of a given link without disrupting traffic under normal operation, as well as multiple loopback modes to facilitate in-field troubleshooting. SAS also supports uninterrupted hot pluggability (surprise removal or insertion of devices). Hot pluggability has become a requirement in the enterprise to enable seamless capacity adjustments without affecting the system's overall operation.
Price and performance
SAS is a high-performance, full-duplex storage protocol that today operates at speeds of up to 12 Gbit/s and is backward compatible to 6 Gbit/s and 3 Gbit/s SAS. These data rates, coupled with the fact that SAS is less expensive than competing technologies, makes SAS a cost-effective option. In addition, when it comes price versus performance, SAS can support not only high-performance SAS devices, but also many of the lower-cost, high-capacity SATA devices used to facilitate the cost-sensitive hyperscale solutions required by large data center deployments.
Scalability brings wide deployment
Ultimately, it was SAS's ability to scale while maintaining unmatched price/performance that moved it firmly into the data center. Multi-initiator support gave SAS the ability to enable numerous hosts and then scale to hundreds and potentially thousands of drives, all while maintaining high reliability, accessibility, and serviceability. This sharing and scaling at new price/performance points enabled SAS to replace Fiber Channel in the data center.
Additionally, SAS addresses unique and emerging requirements within the data center when using SATA storage protocol and nonvolatile memory, such as Flash.
A unique requirement within the data center is the need for unprecedented amounts of capacity-optimized storage. Capacity-optimized storage is driven by client-side economics and is typically based on the Serial ATA (SATA) storage protocol. SATA was never defined to be an enterprise storage protocol, but it can be run under or within SAS. Certain SAS implementations actually support multi-affiliations, enabling SATA to scale and support enterprise-class error handling.
Enterprises are increasingly deploying flash in the data center. Flash has unique performance qualities relative to traditional hard drives, and the SAS protocol continues to add features to optimize for flash while maintaining its overall value proposition. Features such as Persistent Connections and Force Close reduce overall latency. A simplified subset of the SCSI protocol has been defined -- SCSI-SF.
SAS has become the storage protocol of choice within the data center as a result of its enterprise-class features, price/performance point, and scalability. SAS will continue servicing the needs of the data center with its ability to accommodate the SATA protocol and optimize flash solutions.