What drives your data center? Creating graded pools of storage with serial attached SCSI. (Tape/Disk/Optical Storage).

The recent movement to storage area networks (SANs) is being driven by the need to lower the support requirements for managing storage resources and to more effectively utilize storage capacity. IT managers deploying SANs look at storage resources as pools of storage capacity that can be easily allocated across applications as required, lowering support costs and better utilizing the available capacity. However, there are significant differences in the disk drives that can be used to create these pools of storage capacity. ATA drives offer the lowest cost per gigabyte and highest capacities per drive, while SCSI drives provide better availability and performance. Mainstream enterprise applications continue to require the performance and availability of SCSI drives, while new applications are emerging like disk-enhanced backup and hierarchical file storage that will take advantage of the lower costs and higher capacity provided by ATA disk drives.

Serial Attached SCSI will enable IT managers to select one storage system to meet all of these requirements. Disk controllers based on Serial Attached SCSI will connect with both low-cost, high capacity Serial ATA disk drives and with high performance, dual-port Serial Attached SCSI drives. With Serial Attached SCSI, IT managers will be able to create "graded pools of storage" within a single RAID system. These graded pools of storage capacity can be allocated to applications based on each application's requirements for performance, availability, capacity and cost.

Two Classes of Disk Drives to Satisfy Two Market Requirements

Historically, 3.5-inch ATA disk drives have been designed for personal computers and consumer products. Over 150 million of these drives will be sold this year. Users of desktop PCs and consumer applications place a high priority on the capacity per dollar or the "cost per gigabyte" of the ATA disk drive. Striving to provide more capacity per dollar, disk drive manufacturers have invested heavily in increasing the areal density of their disks, with remarkable results. Drive manufacturers are now shipping 3.5-inch ATA disk drives that pack 80GB on a single platter. These investments in improving the areal density have led to significant reductions in cost per gigabyte and dramatic increases in the capacity of ATA disk drives.

In contrast, SCSI disk drives are designed to meet the needs of mainstream servers, workstations and RAID storage systems. These applications emphasize performance and availability over cost. As a result, the cost per gigabyte of ATA disk drives has decreased at a more rapid pace than that of SCSI drives over the past five years. ATA drives are now more than ever the best choice for applications that require the most capacity per dollar.

Disk drive performance can be measured in terms of the number of inputs and outputs per second (IOPS) a drive can sustain. Most SCSI disk drives today spin at 10,000 and 15,000 RPM, while ATA drives typically spin at 5,400 or 7,200 RPM. SCSI drives are designed with faster spin speeds to reduce rotational latency. Lower rotational latency increases the number of IOPS a disk drive can sustain.

To achieve higher spin speeds, disk drive manufacturers have reduced the diameter of the magnetic media. Reducing media size enables the manufacturers to better control the rotational stability of the media and to moderate power consumption. In addition, a smaller diameter reduces the distance the head must travel to position itself over the data. This shorter distance reduces the drive's seek time, which also increases the number of IOPS the drive can sustain.

SCSI disk drives spinning at 10,000 RPM and 15,000 RPM trade off capacity for higher throughput. The maximum capacity of 5400-RPM ATA disk drives is now over 300GB, utilizing platters approximately 95mm in diameter. Four-platter 10,000-RPM and 15,000-RPM SCSI drives now have maximum capacities around 146GB and 73GB, respectively, utilizing platters approximately 84mm and 65mm in diameter. SCSI disks also use sophisticated command queuing and reordering algorithms with multiprocessor architectures to improve the number of IOPS a drive can deliver. Over the past five years, the price per IOPS for SCSI drives has been less than half of what it has been for ATA drives. This makes SCSI drives the choice for applications requiring the best price for performance.

New Applications are Driving the Use of ATA Disks Into the Data Center

While SCSI drives continue to deliver the performance and availability that mainstream servers and RAID storage systems require, new classes of applications are emerging where cost per gigabyte is the most important purchasing criteria. Shrinking backup windows combined with growing capacity requirements are forcing IT managers to seek new ways to perform backup and restore processes. The cost per gigabyte of ATA disk drives has declined to the point where they can become a viable part of these processes. In 1996, the cost per gigabyte of an ATA disk drive was over $100. At the same time, the cost per gigabyte of tape drive media was under $10. Today, ATA disk drives can be purchased for less than $2 per gigabyte while the media for tape drives is priced around $1 per gigabyte. The low cost per gigabyte and high capacity per drive make 3.5-inch ATA disk drives attractive in near-line storage applications like disk-enhanced backup and bulk file storage.

A number of companies have introduced disk-enhanced backup systems based on low-cost, high-capacity ATA disk drives. The faster transfer rates and random access capability of disk drives versus tape dramatically reduce the time required to backup and restore data. These systems create a snapshot or a full point-in-time copy of the data on low-cost ATA disk drives. The time required to copy the data is reduced by the higher transfer rates of disk drives compared to tape. Archival backup to tape is performed from the copy stored on the low-cost ATA drives, while the high performance array goes back on line. Restore processes take advantage of the random access capability and high transfer rates of disk drives to drastically reduce the time required to restore a lost file or directory.

Network attached storage (NAS) systems are using low-cost ATA drives as a means of lowering the cost of bulk file storage. The lower cost per gigabyte and higher capacity per drive makes these systems attractive for storage of infrequently accessed files.

Reducing Complexity

IT managers are increasingly faced with maintaining several classes of storage systems to meet the full spectrum of storage applications required in today's data center. Managing multiple storage systems adds complexity that requires additional management resources and skills. Serial Attached SCSI will reduce this complexity by enabling system managers to meet a wide range of application requirements with a single storage system.

Storage systems based on Serial Attached SCSI will spawn new capabilities. Disk-enhanced backup can be integrated within an array serving mainstream applications. High-availability Serial Attached SCSI drives can be used for online applications while lower-cost, higher-capacity Serial ATA drives can be used for enhancing backup processes. Integrated disk-enhanced backup will lower the cost of these applications by leveraging the electronics, power, and packaging of the online storage to house and manage the low-cost Serial ATA drives used for backup. The hardware costs for implementing disk-enhanced backup will be limited to the cost of adding lower-cost, high-capacity Serial ATA disk drives to the array. Integrating this function into the RAID storage system also means data no longer needs to be copied across the LAN or SAN from the online disk system to the backup disk system. This reduces traffic on the LAN or SAN.

Serial Attached SCSI will also facilitate the development of hierarchical storage solutions that promise to lower the cost of storing static content. New policy-based storage management applications will permit infrequently accessed content to be moved to lower-cost Serial ATA disks. If policies are set to ensure that the content is both infrequently accessed and backed up to disk or tape, the performance penalty and risk associated with using low-cost ATA drives may be acceptable in many end user environments. Serial Attached SCSI will provide a scalable infrastructure that enables cost-effective and manageable deployment of these new hierarchical storage solutions.

Summary

Not all disk drives are alike. ATA disk drives deliver the best cost per gigabyte and highest capacity. per drive while SCSI drives offer the best performance and availability. New applications are emerging, such as disk-enhanced backup and the storage of static content that call for the cost efficiency of ATA disk drives. Serial Attached SCSI will enable one storage system to meet the needs of a wide range of applications. Storage systems based on Serial Attached SCSI will lower support costs by permitting IT managers to create graded pools of storage that will be allocated as needed based on an application's requirements for performance, availability, capacity and cost.

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Kevin Gray is business development manager for Maxtor Corporation Server Products Group (Milpitas, Calif.)

www.maxtor.com