Published: 06 Apr 2005
|SATA pros and cons|
SATA drives deliver storage capacities equivalent to FC/SCSI drives, but for as little as a quarter of the price per gigabyte. SATA drives are also available with much larger capacities than FC or SCSI disks. The major question is whether SATA drives can satisfy enterprise-class availability, data integrity and performance requirements.
SATA evolved from the parallel ATA interface, which has been used mainly in desktop and entry-level server systems. However, the performance and reliability characteristics of ATA drives were simply not engineered for enterprise storage. The initial serial version of ATA (SATA) was developed to overcome some of the limitations of parallel ATA drives. SATA 1.0 delivered improvements in performance (1.5Gb/sec dedicated bandwidth), cabling and reliability (hot plug/ swap). SATA 1.0 drives support speeds up to 10,000 rpm and mean time between failure (MTBF) levels up to 1 million hours under an eight-hour, low-duty cycle. FC drives support up to 15,000 rpm and an MTBF of 1.4 million hours under a 24-hour duty cycle. But SATA's impressive cost advantage over FC has compelled many companies to deploy SATA in secondary and primary storage platforms, even in places where SATA may not yet be appropriate.
Four trends are driving the need for low-cost, high-capacity disk drives:
- Tiered storage
- Shift to disk-based backup
- Regulatory compliance
- Small- and medium-sized business (SMB) requirements
Enterprises are abandoning a one-size-fits-all approach to storage and moving toward tiered storage, where data is migrated to lower classes of storage as its business value depreciates. Many companies have introduced disk into their backup environments in an effort to hit tight backup windows and stringent recovery time/point objectives. Because the ratio of primary to secondary (backup) data in many environments is 1:10, the introduction of disk presents a number of economic challenges requiring a low-cost, high-capacity disk solution.
New regulatory compliance requirements mandate that businesses preserve electronic records for many years. Moreover, some of these regulations define specific recovery time objectives for data, which often means the data must be retained online. The explosive growth of online archive data sets, often into petabytes, is another driver for less-expensive disk drives.
Single point of failure
Enterprise-class data requires two independent, end-to-end data paths from the client application to the back-end disk drives. A single point of failure along the data path will compromise the entire storage architecture. While FC-based configurations ensure high availability with dual-ported drives and pairs of redundant FC loops with access to all of the drives, SATA provides only a single-ported drive.
In block-based storage, user data is stored in 512-byte blocks with disk drives typically formatted to fit one block into each drive sector. For additional data integrity, block size can be expanded to 520 bytes, with the additional 8 bytes used to protect the data in the 512-byte block. Some FC apps require drives to be formatted with a media sector size of 520 bytes. SATA drives can only be formatted with a media sector size of 512 bytes.
To overcome deficiencies in Port Bypass Circuits--rudimentary components that enable FC Arbitrated Loop (FC-AL) traffic to bypass a failed drive and continue operations--FC-AL switch devices have been enhanced with component-level diagnostics that can effectively diagnose and isolate the activity of individual FC drives. SATA-based architectures require similar or higher levels of diagnostics, particularly given their lower MTBF characteristics; SATA on its own lacks this capability.
|When to use SATA|
- NL35 (Seagate Technology Inc.). Seagate's NL35 drive family is a high-capacity, hybrid drive design that 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|
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.