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

Computer Technology Review, April, 2003 by Kevin Gray

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.