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Quantum Bigfoot: Shaping The Future Of Storage

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A Quantum White Paper

Introduction


While the desktop PC market is a rapidly changing environment, there are some identifiable trends that have remained consistent. User demand for additional computational power at lower cost continues to drive the development and adoption of increasingly powerful microprocessors. Similarly, demand for higher-capacity, lower-cost-per-megabtye hard disk drives steadily pushes advances in recording technology.

In response to the market's need for more (and less expensive) disk drive storage, Quantum designed a specific family of products. Called the Quantum Bigfoot series, these 5.25-inch disk drives offer an attractive balance of high capacity and value for the storage-intensive consumer PC market. Several important factors in the PC market are fueling the push for more megabtytes (MB): Microsoft Windows 95 usage, multimedia applications and files, more sophisticated graphics and Internet file downloading. These market forces are expected to continue to challenge mass storage suppliers to provide higher capacity at a lower cost per megabyte.

For Quantum's major OEM customers, which include eight of the world's top ten desktop system manufacturers, the unique Quantum Bigfoot drive is an excellent solution ó a high-capacity, economical product that leverages many field-proven design elements from Quantum's high-volume 3.5-inch disk drives.

The ultra-slim drive, which measures 8 by 5.75 by .75 inches, can be housed in a majority of PCs without any customization to system enclosures, fitting easily into unused hard drive and CD-ROM bays. Power and interface connections are identical to 3.5-inch drives.

Most systems shipped in the last year provide one or more vacant 5.25-inch half-height (1.63 inch) drive bays. For example, out of 556 non-server PCs reviewed by publications between June 1994 and May 1995, 85 percent of them featured at least one available 5.25-inch slot. The number of available bays in general-purpose PC systems is even higher: 97 percent of 165 systems reviewed in December 1994 had three or more available 5.25-inch slots. So it's clear that physical space will not limit the usage of a 5.25-inch drive like Quantum Bigfoot.

When used as the primary storage device, Quantum Bigfoot drives exceed most consumers' current expectations for capacity, and provide room for expansion. With either 1,280MB (one disk) or 2,550MB (two disks) available, the drives' capacities match up well with the multimedia capabilities of today's consumer PCs. The Quantum Bigfoot drives are also expected to see usage as add-on units for PC users who want to augment their existing storage capacity.

The new family of drives satisfies users in another way. Today's PC buyers are very sensitive to the rapid pace of technological change. When considering the purchase of new systems, the fear of obsolescence is often a consideration. Buyers are likely to spend extra money on a system that offers capabilities beyond what is immediately needed. PCs equipped with Quantum Bigfoot drives provide extra assurance by having plenty of storage capacity for future growth.

5.25-inch form factor performance considerations

Because Quantum Bigfoot is a 5.25-inch device, its form factor has a favorable impact on drive performance, relative to 3.5-inch drives. Since the drive uses larger media, more information is stored on any given data track, resulting in better sequential performance.

For example, for the 3.5-inch Quantum Trailblazer™ 850 drive, every transaction over 71 kilobytes (KB) involves at least one head switch and possibly a seek. The Quantum Bigfoot drive, with its greater capacity per track, can handle up to 138KB transfers without switching heads or seeking, reducing these time-consuming operations. This is an important consideration for long, sequential transfers that take place in applications such as multimedia playback.

The remainder of this paper provides a detailed comparison of three specific aspects of Quantum Bigfoot performance with typical 3.5-inch drive performance: rotational speed, random seeking and sequential read or write performance.

Rotational speed

Rotational speed (RPM) is often cited as a type of shorthand for two aspects of performance, data rates and latency. (Data rates indicate how fast data can be transferred on or off the disk, while latency is the average time read/write heads wait for data to rotate into position after a particular data track is located.) Using RPM as an indicator of performance is accurate as long as form factors are identical. But, as demonstrated below, a 3,600 RPM 5.25-inch drive can offer data rates equivalent to a 5,400 RPM 3.5-inch drive.

To begin, it's important to understand that there is a close relationship between peak data channel rates and rotational speed ó especially as drive performance and capacities grow. In fact, the selection of a particular data channel integrated circuit (IC) and the data rate it is capable of handling actually determines the maximum rotational speed of the drive mechanics. That's because the data rate is directly determined by the speed of the bits passing by the recording heads; the faster the drive spins, the more quickly data goes by the heads.

For the Quantum Bigfoot product, the design criteria were centered around balancing maximum capacity and attractive costs. For maximum capacity, a 5.25-inch drive was chosen because it provided 90 percent additional recording area over a 3.5-inch disk. To minimize cost, a field-proven Partial Response Maximum Likelihood (PRML) read channel IC was chosen. The chip's design could reliably support internal data rates up to 89 megabits (Mb) per second. These two design choices combined to determine an "appropriate" rotational speed: 3,600 RPM.

It is interesting to note that the read channel used in the new Quantum Bigfoot family was first used in a family of 3.5-inch form factor Quantum drives called Quantum Fireball™. Since the circumference of the outer track on the 3.5-inch disk is smaller, the rotational speed of the drive could be increased to 5,400 RPM and still be supported by the same read channel.

How can a 5,400 RPM 3.5-inch drive (Quantum Fireball) and a 3,600 RPM 5.25-inch drive (Quantum Bigfoot) have similar data rates? Because of the larger diameter of a 5.25-inch drive (spinning at 3,600 RPM), head velocity over the media at its outside diameter is the same as a 5,400 RPM 3.5-inch drive. This factor, coupled with the advanced read channel, results in a data rate equivalent to that of the smaller 3.5-inch drive.

As is evident, RPM by itself is not an accurate measurement of performance across different disk drive form factors. A true performance comparison should look at all the factors that affect a drive's ability to transfer data.

Random seek performance

Similar to rotational speed, seek specifications are not easily compared across form factors, even if drive technology is otherwise identical. Geometry affects seek behavior, and adjustments are required to the seek time specifications of 5.25-inch drives for accurate comparisons with 3.5-inch drives.

Quantum Bigfoot's increased data area results in an actuator stroke distance that is 30 percent longer than 3.5-inch drives. Assuming that the actuators of the 3.5- and 5.25-inch drives move at the same velocity, the longer stroke of the 5.25-inch drive results in a higher seek time specification because the 5.25-inch drive's actuator travels farther. Simply comparing absolute seek times "penalizes" a 5.25-inch drive for providing more data.

The larger 5.25-inch drive does offer a significant seek time advantage in another way. Assuming similar linear bit densities, the outer tracks of the 5.25-inch drive contain significantly more data per track. As a result, when transferring files, fewer seeks (and head switches) are needed, improving throughput.

As seen with rotational speed, evaluating drive performance solely on the basis of random seek times may lead to an incorrect conclusion, particularly when two different form factors are involved.

Sequential read or write performance

Sequential read or write performance - important for full-motion multimedia playback or when using large, contiguous graphics files- is generally better with a 5.25-inch drive such as Quantum Bigfoot.

For any given transaction, a 3.5-inch drive will have to work harder because of its shorter data tracks. With a 5.25-inch form factor drive, more data is contained in any given track and, for sequential transactions, reads and writes can occur with less head motion and fewer head switches.

Looking at a specific example, let's compare a two-disk 5.25-inch drive and a two-disk 3.5-inch drive during a 1MB sequential read operation. As can be seen in the description and figure on the next pages, the larger 5.25-inch drive could save the time it would take to do six head switches and two combination seeks/head switches (moving between tracks and switching disk surfaces). Including the effect of rotational speed, the 5.25-inch drive saves about 41 percent of the read time, or about as much time as five full-stroke seeks on the 3.5-inch drive.

Sequential Read Timing Comparison

(3.5-inch drive versus 5.25-inch Drive)

Notes:

(1) This comparison was based on actual product specifications: the 3.5-inch drive used is a Quantum Trailblazer 850AT. The 5.25-inch drive used is a Quantum Bigfoot 2.5AT.

SpecificationsTrailblazer 850ATBigfoot 2.5AT
Form Factor (inches)3.5 5.25
Drive Capacity (MB)850 2,577
Disks/Heads2/4 2/4
Track-to-track seek time (ms) 53.5
Rotational Speed (RPM)45003600
Rotational Latency (ms)6.678.33
Head Switch Timing (ms)4.53.0


(2) Calculations measured the total time for an internal (disk-to-buffer) read of 1000KB (1MB)

(3) Outer tracks for both drives are used. Quantum Trailblazer 850 track capacity is 71KB; Quantum Bigfoot 2.5 track capacity is 138KB

(4) Timing starts when heads are on proper track for file to be read (e.g. no initial seek)


Trailblazer 850AT (3.5-inch) completes the sequential read in 264.0ms

Summary of Head Activity

15 full track reads

>11 head switches

3 single-track seeks

Bigfoot 2.5AT (5.25-inch) completes the sequential read in 154.8ms

Summary of Head Activity

8 full track reads

6 head switches

1 single-track seek

A 5.25-inch drive such as Quantum's Bigfoot 2.5AT would complete a 1MB sequential operation 109.67ms faster (41 percent faster). If large files can be contiguously located, the read or write performance of a 5.25-inch drive will outperform many 3.5-inch drives.

 

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Summary


In response to market demands for high capacity, economical hard disk drive storage, storage manufacturers are beginning to return to the 5.25-inch form factor. The Quantum Bigfoot family of 5.25-inch products are the first in what is expected to be a popular choice for consumer PCs. In addition to providing high capacity and excellent cost-per-megabyte economics, the drives help allay consumers' fears of storage obsolescence.

Evaluating the performance of this new class of drives requires a more detailed look at both specifications and actual system applications. Simply comparing specifications across form factors may lead to incorrect assumptions about "real world" data transfer performance. The 5.25-inch form factor, because of its larger track capacity, can offer superior sequential read/write performance when compared to many 3.5-inch drives.

See Also