Choosing the Right Drive: How to Find the Sweet Spot in Storage

Authors: Franco Castaldini, Sr. Product Marketing Manager and
                   Kent Bransford, Sr. Technical Editor
                   Seagate Technology

“Efficiency” is the watchword in today’s enterprise as it strives to achieve the optimal blend of performance and cost-effectiveness. To be sure, this can be a delicate balance. Investing in unnecessarily powerful (and costly) solutions is wasteful, yet deploying inexpensive (and underpowered) solutions can seriously degrade system performance and overall productivity. In either case the bottom line ultimately suffers.

Fortunately, the enterprise’s heightened focus on efficiency coincides with an unprecedented variety of storage options from which to choose. Spearheaded by Serial Attached SCSI (SAS), a comprehensive range of serial-based solutions now enables savvy IT managers to hit the “sweet spot” of performance and cost-effectiveness by specifying the most appropriate storage device for any given application.

Storage Selection Criteria
To determine which serial disk drive is the most appropriate solution for a specific application, four evaluation criteria can be applied:

  • Performance
  • Reliability
  • Scalability
  • Price

In online/transactional environments where mission-critical data must be delivered with high availability, SAS and Fibre Channel (FC) are clearly the best choices. Utilizing the proven SCSI command set, both of these enterprise-class drives offer exceptional throughput (SAS delivers 3.0 Gb/s, with a clear roadmap to 12.0 Gb/s), ensuring data availability under heavy loads (e.g., multiple, concurrent users). By contrast, Serial ATA (SATA) is a desktop-class drive, designed and manufactured to cost-effectively meet the lighter-duty requirements of that environment. Lacking the robust hardware and advanced command queuing of FC and SAS, SATA’s performance severely degrades as the number of users attempting to access data increases.

High data availability is not only dependent on performance, but is also a function of minimizing system downtime due to drive failures. SAS and FC are specifically designed for the rigors of enterprise use, carrying MTBF ratings in excess of one million hours and warranties up to five years in length. To further provide for high availability and greater uptime, SAS’ dual data ports ensure that if one host controller fails, the extra data port can maintain uninterrupted communication with a second controller. Furthermore, SAS and FC drives safeguard data integrity via their comprehensive verification/error correction capabilities. Reflecting their desktop origins, SATA drives typically carry MTBF ratings barely half those of SAS and FC, and in some cases offer only a one-year warranty. SATA’s data integrity provisions are similarly modest.

Enterprise storage solutions must be able to seamlessly grow as needs change and both SAS and FC offer extraordinary levels of scalability. FC is most appropriate for large enterprises that deploy remote-connect storage solutions such as SANs in physically disparate locations. SAS is designed for direct-connect environments where servers and storage solutions are located in relatively close proximity (e.g., a common data center). Through its use of high-speed switches (known as expanders), SAS enables over 16,000 devices to be connected in a single domain.

Conversely, SATA’s scalability is severely limited, though the new SATA II standards specify devices (known as Port Multipliers) that can offer a modest degree of expandability. Fortunately, SAS offers an elegant workaround to this dilemma. Its compatibility with SATA enables SATA devices to be freely mixed with SAS devices in a SAS domain, thus vastly increasing SATA’s ability to scale when needed.

While FC drives are similar in cost to SAS drives, the necessary FC infrastructure is significantly more expensive than its SAS counterpart. FC is therefore better suited to large enterprises that can justify the substantial investment required. As the enterprise’s emphasis on efficiency grows, IT managers are investigating the economies of data center rationalization. This entails paring down a multitude of decentralized data centers (and servers they house) to fewer, smaller and more centralized centers while simultaneously consolidating the storage systems within to achieve optimal performance and cost-effectiveness. SAS delivers throughput and reliability comparable to FC, but at notably lower cost, making it ideally suited to such a strategy.

SATA, of course, offers exceptionally high capacity per dollar. In environments where a great deal of non-mission-critical data is infrequently accessed (e.g., near-line storage), SATA is a highly cost-effective solution. And its compatibility with SAS enables both performance and capacity needs to be addressed by a single SAS enclosure, enabling further cost savings.


SAS and SATA Synergies
SAS cables/connectors, backplanes and host bus adapters (HBAs) are fully compatible with SATA drives. Furthermore, SAS employs three different protocols to transport information over its serial interface, one of which (Serial ATA Tunneling Protocol, or STP), enables SAS controllers to seamlessly identify and communicate with SATA devices.

The result? SAS provides the path through which deployment of SATA in the enterprise offers the greatest strategic benefit–the freedom to select the optimal disk drive for a given task. SAS drives are clearly the best choice for mission-critical enterprise use where transactional/online performance and reliability are crucial, while SATA drives are cost-effective for lighter-duty use such as near-line and backup/restore storage. SAS and SATA drives can be consolidated on a common backplane and housed in a single enclosure, enabling one SAS-based subsystem to handle the full gamut of enterprise storage duties. This can save substantial amounts of money and space, as well as reducing IT storage maintenance costs.

Maximum storage efficiency is achieved by deploying the most appropriate storage device for each type of enterprise data. With the addition of SAS, IT professionals now have an extensive array of targeted solutions at their command, ensuring that they can consistently hit the “sweet spot” of performance and cost-effectiveness.

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