Are SATA drives ready for the enterprise?


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When to use SATA
Disk drive vendors and other storage companies have been developing alternative systems and architectural strategies to overcome the limitations of SATA drives.

  • NL35 (Seagate Technology Inc.). Seagate's NL35 drive family is a high-capacity, hybrid drive design that

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  • comes in two flavors: a single-ported SATA interface and a dual-ported FC interface. The NL35 family overcomes the limitations of SATA, such as port redundancy, but has a number of drawbacks. For example, the NL35 is a niche drive technology developed by a single disk drive manufacturer, Seagate, for Hewlett-Packard Co. The NL35 is also very pricey vs. other SATA drives; a 250GB SATA drive is approximately $262, while an NL35 drive with an FC interface and equivalent capacity is $695.
  • SAS/SATA combination. Serial Attached SCSI (SAS) is a serial replacement for parallel SCSI. The SAS interface delivers the benefits of SCSI, most notably better data rates than SATA. Like its parallel predecessor, SAS can be used as the interface to the storage controller and high-performing disk drives. Although this type of architecture addresses some of the scalability limitations of SATA, given its improved performance, it doesn't fully resolve a number of key issues required to make SATA enterprise-ready. First, an architecture combining SAS/SATA will still have a single point of failure at the drive level because the SATA drives remain single ported. Second, SAS technologies don't seamlessly integrate with users' existing investments in FC array and SAN technologies, although SAS-FC bridging products will become available.
  • Board-based FC-SATA. A number of storage arrays, such as EMC Corp.'s Clariion and Hitachi Data Systems Inc.'s Thunder, used OEM-supplied, board-based solutions to allow the intermixing of FC and SATA drives within the same system. This is a classic first-generation approach to enable interoperability between heterogeneous interface standards. These solutions address the single-ported drive issue using port selectors. A port selector is a 2:1 multiplexer chip placed on a small interposer card that fits inside the disk canister and becomes part of the SATA disk drive field-replaceable unit. Typically, the card has a SATA connector on one side and an SCA-2 connector on the other, and provides dual-port capability. While this architecture gives users flexibility in the choice of a back-end drive interface, board-level solutions add significant cost to the array, aren't optimized for performance and create cooling challenges.

Standard SATA, coupled with low-cost, chip-based FC-SATA bridging technology, is another way to kick SATA up to the enterprise-class level. The bridging technology allows SATA drives to emulate FC drives, and to plug seamlessly into FC-based storage systems consisting of FC controllers and FC-AL loops to the JBOD. This configuration dual ports the SATA drives and delivers end-to-end redundant data paths.

By distinguishing and translating between user sectors (520 bytes) and media sectors (512 bytes), bridging technology lets SATA drives exhibit the data integrity features of performance-optimized FC drives. Bridging chips can also improve the fault tolerance of SATA drives by providing drive diagnostics and failure identification.

The combination of SATA and low-cost, chip-based bridging technology has significant merit. It addresses the inherent technology limitations of the standard SATA interface and makes SATA technology applicable for a variety of primary and secondary storage apps. Moreover, it protects enterprise investments in FC platforms and provides a seamless path for the introduction of SATA without massive capital outlays and forklift upgrades. Finally, because the technology emulates FC using SATA drives from any vendor, it delivers significant flexibility.

A sampling of SATA products
SATA applications
For secondary storage, SATA drives are ideal repositories for disk-based backups and online archives. In each case, users must grapple with massive data sets on the order of petabytes and data growth at or above 100% per year. Disk media performance, though not negligible, isn't as critical as it would be in many production application environments.

Data for critical applications, particularly those sensitive to I/O latency, is best stored on performance-optimized SCSI or FC drives. For other categories of application data that are business- rather than mission-critical and aren't performance-sensitive, SATA with enterprise-class extensions is a natural fit. Examples of these applications may include data mining/warehousing, unstructured content repositories, e-mail in certain environments and aged mission-critical data.

Because the cost advantages of SATA are huge compared to FC drives, a storage admin may be tempted to deploy SATA in environments where the technology may not yet be appropriate. Look for solutions, albeit more costly than plain SATA, that add an extra measure of reliability and protection. Companies should favor SATA solutions that incorporate low-cost, chip-based bridging technologies and preserve existing investments and expertise with FC-based products. While chip-based bridging technology narrows the price gap between SATA and FC drives, the improvements in reliability, performance and data integrity put all of the pieces in place for successful enterprise SATA deployments.

This was first published in April 2005

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