Serial ATA takes the storage world by storm

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Serial ATA is making in-roads into the corporate arena, an area previously dominated by Fibre Channel and SCSI hard drives. The heady mix of low cost per gigabyte and intelligent controllers make it an attractive solution for data centres and server rooms alike -- argues Steve Gardner, Director, Product Marketing at Engenio Information Technologies.

The barriers between high-end and low-end storage are crumbling. Serial ATA (SATA) hard drives are now offering high throughput rates and connectivity previously only reserved for SCSI or Fibre Channel disks. Thats good news for users - a technology with a lower cost per gigabyte is just what they have been waiting for.

SATA itself has long pointed the way ahead in the consumer market - even mid-range mainboards are now equipped with SATA connections - and a study by market research company iSupply predicts that by 2005 the parallel bus will no longer feature on new systems.

However, history has shown that the adoption of new technology is often slow within in the enterprise arena. Concern over failure rates and performance levels has dominated debate on the new serial bus standard. But this is being proven unjust and the widespread view is that Serial ATA will prevail. But why?

First and foremost, administrators have limited choice in the matter as more and more data is created and demand for instantaneous availability grows. Lawmakers in the U.S. - as well as the European Union - bear no mean share of the responsibility for this by producing stringent rules governing how long data must be held and the speed of its retrieval. That doesnt present a huge problem for information that is constantly in demand it is in any case, stored on fast mass-storage systems that guarantee access at all times.

As things stand, it is the older, less-used data, meanwhile, that is usually migrated off to slower, and therefore cheaper, archive media.

But, in a growing number of scenarios, what may appear sensible from a cost point of view no longer fits the bill in terms of access time and availability. Storage systems with Serial ATA deliver on both. On the one hand, they offer considerably faster access to stored data than tape media. On the other, they cost significantly less than systems with SCSI or Fibre Channel hard drives.

Security with RAID
ATA technology was originally developed as a cheaper alternative to SCSI for use in desktop PCs. In contrast to server based operations, desktop computers are generally not permanently on and the load on hard drives is thus not as great.

Serial ATA is similar in many respects to its predecessor. The instruction set and design (the controller logic is located in the computer rather than on the disk) are closely related. Based on these similarities, users have questioned whether magnetic media with a Serial ATA interface is really suited to demanding server applications. However at its rated duty cycle, SATA is just as reliable as Fibre Channel as both drives have a rated MTBF (mean time between failure) of over 1 million hours.

Owing to some reservations, iSupply is only able to forecast a market share for SATA in the enterprise area of just 30% by as late as 2007. But in most practical cases, the worries bear no relation to the reality. Server media almost always feature in an array. Powerful RAID controllers ensure redundancy and protect user data against drive failures. Storage systems from Engenio, available through its OEM partners, can control up to 224 SATA disks and combine them at mixed RAID levels. All primary components such as power units, fans and controller modules are designed redundantly and can equally be exchanged during operation. In terms of availability, the storage system is no different from a system made up of SCSI or Fibre Channel drives.

The debate on duty cycles and MTBF does not as such imply that SATA hard drives are more prone to crash than other technologies.

Engenio's experience appears to suggest that SATA media must first endure an intensive burn-in process. Once accomplished, failure rates are equivalent to those of Fibre Channel and SCSI. This makes it the duty of the storage system manufacturer to ensure a long life for the medium by way of intensive tests and certification. At Engenio, new drives have to withstand a 24-hour endurance test under tightened conditions. In addition, Engenio only uses Serial ATA hard drives that have been released by the maker for permanent 24-hour operation.

The correct use of the SATA drives is fundamental. Lifecycle and failure rates are always indicated in reference to a duty cycle, which determines the assumed utilisation of a drive. But far from providing the recommended ratio of operational time over 24 hours, the duty cycle figure defines the ratio of disk access involving the drives mechanical parts (i.e., search, read and write operations) spread over 24 hours. The time the drive spends merely spinning is not counted. This results in applications eminently suited to SATA drives, and others that more appropriate to the Fibre Channel and SCSI domains. All applications that feature a high number of non-sequential transactions are better off with SCSI or Fibre Channel as the great number of mechanical operations needed to permanently reposition the heads drives up the duty cycle. Speed in transaction-intensive applications also favours Fibre Channel or SCSI. These disks generally spin at 15,000 or 10,000 rpm as opposed to 7,200 rpm for SATA drives.

SATA technology comes into its own in areas of use where a few hard drives should achieve the highest possible capacities. Serial ATA disks of 250 GB are currently available, with the next doubling of capacity around the corner with 400 GB lined up.

SATA is ideally suited for secondary storage (including disk-to-disk backup and storing fixed content or reference data, and now entry-level storage (which has low cost and low I/O requirements).

SATA is also ideally suited to disk-to-disk backups which involves replacing magnetic tapes with an economical RAID array as a high-capacity storage medium. RAID arrays and a casing featuring redundant power supplies and fans provide data security.

Disk-to-disk backups are now all the rage. More and more backup applications support hard drives as the target media for data backups, with the backup concept integrating specific advantages of disks such as multiple, parallel data streams and virtually instantaneous access to any file. Whats more, it automates the migration of older data to lower-cost SATA disks. Software with these functions is referred to as next-generation backup.

Scalable storage systems
Serial ATA and Fibre Channel/SCSI storage systems currently feature in data centres as separate units. This is likely to remain so for some time according to analyst Randy Kerns from The Evaluator Group. At present, Serial ATA is considered secondary storage, used in data centres either for online/near-line archiving or for other tasks above and beyond business-critical company data. Yet, compared with the costs savings achieved by less expensive SATA drives, the combination of SATA with other disks in a storage system can reduce the long-term administrative effort involved in directing the data to the right part of the system to a far greater extent. Nevertheless, depending on the size of the array, the cost advantage of opting for SATA can be considerable. Storage media can make up to anywhere between 30 and 85 per cent of the total costs of a storage system. Costs per array can be halved if SATA is chosen.

Despite the continued rise in the cost of external storage, IT departments are under pressure to deliver maximum performance on limited budgets. A SATA-based storage system can go a long way to remedying these constraints. Manufacturers are already reacting to the development with powerful starter systems. Engenio, for instance, offers a new basic model that integrates SATA interface chips as a permanent controller feature to its OEM partners. This enables up to 14 SATA drives to be configured in the initial 3U enclosure along with the controllers. With a redundant configuration, the 2822 disk technology already comes with capacity of 3.5 TB in the basic model. The system demonstrates to customers that entry-level systems need not be any less reliable and come complete with many functions. With Fibre Channel Interface Links, the system is capable of scaling to far higher capacities.

Engenio are exhibiting at Storage Expo the UK's largest and most important event dedicated to data storage, now in its 5th year, the show features a comprehensive FREE education programme, and over 90 exhibitors at the National Hall, Olympia, London from 12 - 13 October 2005 www.storage-expo.com

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