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CONTENTS

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Mid-size businesses and branch offices using departmental servers with storage

requirements of 24 GB or less and running Microsoft Windows NT®, Novell

NetWare®3.x or IntranetWare®4.0 have a growing need for high performance, high

capacity, and reliable backup methods. Digital Data Storage-3 (DDS-3) technology,

which is the recording format used by Digital Audio Tape (DAT) products, meets this

need with three times the capacity and twice the data transfer rate of DDS-2

technology and offers improved automated head cleaning mechanisms that enhance

backup reliability. DDS-3 also provides low total cost of ownership for customers,

due to low-cost media and backward read/write compatibility with existing DDS-1

and DDS-2 media.

This brief discusses DDS-3 technology and the Compaq 12/24 gigabyte (GB) DAT

Drive, which is designed with DDS-3 technology.

Please direct comments regarding this communication to the ECG Technology Communications Group at this

Internet address: Te[email protected]

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ECG036/1297

NOTICE

The information in this publication is subject to change without notice and is provided “AS IS”

WITHOUT WARRANTY OF ANY KIND. THE ENTIRE RISK ARISING OUT OF THE

USE OF THIS INFORMATION REMAINS WITH RECIPIENT. IN NO EVENT SHALL

COMPAQ BE LIABLE FOR ANY DIRECT, CONSEQUENTIAL, INCIDENTAL,

SPECIAL, PUNITIVE OR OTHER DAMAGES WHATSOEVER (INCLUDING WITHOUT

LIMITATION, DAMAGES FOR LOSS OF BUSINESS PROFITS, BUSINESS

INTERRUPTION OR LOSS OF BUSINESS INFORMATION), EVEN IF COMPAQ HAS

BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

The limited warranties for Compaq products are exclusively set forth in the documentation

accompanying such products. Nothing herein should be construed as constituting a further or

additional warranty.

This publication does not constitute an endorsement of the product or products that were

tested. The configuration or configurations tested or described may or may not be the only

available solution. This test is not a determination of product quality or correctness, nor does

it ensure compliance with any federal, state or local requirements.

Compaq is registered with the United States Patent and Trademark Office.

Microsoft and Windows NT are trademarks and/or registered trademarks of Microsoft

Corporation.

Novell, NetWare and IntranetWare are registered trademarks of Novell, Inc.

Cheyenne, ARCserve, JETserve, and JETserve Plus are registered trademarks of Computer

Associates International, Inc.

Seagate and Seagate Backup Exec are registered trademarks of Seagate Software.

Other product names mentioned herein may be trademarks and/or registered trademarks of

their respective companies.

DDS-3: Enhancing DAT Reliability, Capacity, and Performance

First Edition (December 1997)

Document Number ECG036/1297

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ECG036/1297

INTRODUCTION

An efficient, reliable, and cost-effective solution for backup of critical data is a requirement

for companies of any size. Compaq has a strong record of providing industry-standard storage

solutions to fit a variety of customer needs. Not everyone requires hundreds of gigabytes of

storage or can afford the more expensive backup solutions designed for customers with much

heavier backup loads. Increasingly, small and mid-size businesses use departmental servers,

such as the Compaq ProLiant 1200, ProLiant 1600, or ProLiant 2500 servers with storage

requirements of 24 GB or less. To meet the needs of mid-size businesses, Compaq has an

affordable line of DAT drives that use the DDS recording format and provide fast, reliable

data storage.

This brief explains DAT and DDS technologies and the performance improvements of DDS-3

over DDS-2. This paper also explains why DDS-3 provides low total cost of ownership.

Finally, this brief describes the Compaq 12/24 GB DAT Drive with DDS-3 format, including

the backup software that it supports.

BACKGROUND INFORMATION

DAT technology was initially developed for consumer audio recording. DDS is a data storage

format that was developed from DAT to provide high-density, high-quality, reliable computer

data backups. The physical media (drive and tapes) are referred to as DAT, while the

recording format is referred to as DDS.

What is DAT?

DAT is a digital recording technology that uses 3.81mm (0.15-inch) wide tape in a helical

scan read/write recorder. Unlike other tape technologies that only have tape movement,

helical scan has both tape and head movement. With tape and head movement, the tape speed

is slower, incurring less tape wear and maintaining better control of the tape as it moves

through the tape path. The read and write heads in a drive that uses helical scan are attached to

a rotating transport drum, which sits at a 6-degree angle to the tape media (Figure 1). Data is

recorded in diagonal stripes from bottom to top on the tape, which moves in the same direction

as the drum.

Figure 1: DAT helical scan recording.

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ECG036/1297

The drum in a DAT drive incorporates two read and two write heads, which are diametrically

opposite one another (Figure 2).

Figure 2: Top view of drum and diametrically opposite read and write heads.

Because the write heads in a DAT drive are set at diametrically opposite angles, they record

data on adjacent tracks at angles that vary by +20°and -20°(Figure 3). Recording data in

diagonal stripes greatly minimizes magnetic crosstalk. Magnetic crosstalk is the interaction

between signals from different tracks.

Figure 3: Adjacent tracks are recorded at opposite angles, minimizing magnetic crosstalk.

Compaq DAT Drive Product Line

From the DDS format specification, Compaq developed a series of DAT drives using DDS

technology, the first of which was the 2/4 GB DAT Drive (DDS-1), followed by the 4/8 GB

DAT Drive (DDS-2). With the advent of DDS-3 technology, Compaq developed the 12/24

GB DAT Drive, which has a capacity of 12 GB native (uncompressed) and 24 GB

compressed.

Tape Direction

Tracks

Write head

Read head

Write head

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ECG036/1297

ADVANTAGES OF DDS-3

In comparison to DDS-2, DDS-3 technology provides significant improvements in capacity,

transfer rate, and reliability. When considering all the new features, the drive price, the media

price, and the overall reliability, it is easy to see DDS-3 also offers a low total cost of

ownership.

Increased Capacity

Although DDS-3 cassettes use almost the same length of tape as existing DDS-2 cassettes,

DDS-3 has three times the native capacity (12 GB vs. 4 GB). Factors contributing to this

increased capacity include:

•Increased linear bit density

•Increased format efficiency

•Increased use of track space

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DDS-3 was originally intended to have a native capacity of 8 GB, twice that of DDS-2.

However, the development team discovered that the linear bit density could be increased more

than they originally thought. In fact, the density was doubled from 61,000 bits per inch (bpi)

to 122,000 bpi. While this doubling of the linear bit density does not result in double the

capacity, this increase does significantly impact the capacity of DDS-3.

The capacity of a tape is almost doubled with data compression, depending on the nature of the

data being compressed. Typically, text can be compressed at a 2:1 ratio, while graphic files

can be compressed at a 5:1 ratio. However, a file that is already compressed will not compress

further during the backup.

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DDS-3 also has increased format efficiency over DDS-2 due to Partial Response Maximum

Likelihood (PRML). PRML is a technique used to differentiate a valid signal from random

noise. Noise results in data errors. Because DDS-3 has PRML, it incurs fewer data errors due

to random noise.

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In multiple ways, DDS-3 technology increases the amount of track space that can be used to

store data. First, DDS-3 uses the timed tracking servo format instead of the embedded servo

format used in DDS-1 and DDS-2.

Servo tracking is a scheme used to regulate positioning and seeking operations in disk and tape

drives. This scheme relies on positioning data recorded on the media surface to generate

correction signals for the servomechanism. A system with an “embedded” servo, as in the

Compaq 2/4 GB DAT and 4/8 GB DAT drives, uses the automatic track following (ATF)

method to ensure the tape head is in the center of the track being read. There are ATF areas

embedded at the beginning and end of each track; user data is recorded between them. The

read and write heads are wider than a single track, so the head reads the ATF areas from the

two tracks on either side, as well as that of the track the head intends to read. By comparing

the content and strength of the signals from the ATF areas, the servo system adjusts the drum

and tape and allows the head to pass directly over the centerline of the track being read (Figure

4). The ATF areas of the track cannot be used to store data.

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ECG036/1297

Tape Edge

Tape tracks

ATF area

Data area

Read/write head

Tape tracks

Figure 4: By reading ATF information from tracks immediately left and right of the

primary track it is reading, the head centers its position on the track.

The Compaq 12/24 GB DAT Drive does not use embedded servo tracking with ATF. Instead,

it uses a timed tracking scheme that frees the ATF areas for user data, thus increasing the

storage capacity.

Timed tracking is a track-following servo format that provides servo input by measuring the

time required for read or write heads to scan from the tape edge to a known reference point on

the track. These reference points are equally-spaced “timing marks.” By referencing from the

edge of the tape, the drive is able to measure the time it takes to reach the timing mark (Figure

5). Any deviation in time indicates a deviation from the track centerline. If the timing is too

early, the tape speed increases to center the head on the track; if it is too late, the tape speed

decreases. This process increases efficiency and performs head/track alignment more

frequently than the embedded servo format. The timed tracking format does not require ATF

storage on the track, so more of the track is available for storing user data.

Tape Edge

Early Late

"On-time"

Timing mark

Figure 5: Timed tracking servo.

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ECG036/1297

Other ways DDS-3 technology increases track space are by compressing sub-code information

into less space and by packing user data into fewer, larger blocks, thereby reducing total

overhead for data blocks.

As a result of the increased linear bit density, format efficiency, and increased use of track

space, DDS-3 has three times the track space that DDS-2 has available for data storage instead

of timing, sub-code, Reed-Solomon ECC, and block overhead (Table 1).

Table 1: DDS storage capacity comparison.

DDS Version Native Capacity

(uncompressed) Compressed Capacity

DDS-1 2 GB 4 GB

DDS-2 4 GB 8 GB

DDS-3 12 GB 24 GB

Faster Transfer Rate

DDS-3 has a maximum native data transfer rate of 1 MB/s (2 MB/s with compression), which

is nearly double that of DDS-2. This increase is due in part to the change from embedded

servo format to the more efficient timed tracking format. The increase in frequency/bit density

also plays a part in the faster transfer rate of DDS-3. Because (1) the track is the same length

for DDS-2 and DDS-3, and (2) there are twice as many bits in the same length of tape, the

head passes over twice as much data in the same amount of time.

Fast search is another feature that increases the data transfer rate. Fast search is the process of

reading just the sub-code areas to locate particular data on a tape. The sub-code areas include

information about user data such as group count, filemark count, setmark count, and block

count, which allows the drive to find a particular data group quickly. The tape heads access

the sub-code at a speed 100 times that of the normal read/write process. Normal tape speed

for DDS-3 is 10.4 mm/s, but in fast search mode, DDS-3 tape speed is 1040 mm/s.

During fast search, the rotational speed of the drum is slightly reduced so that the tape passes

over the heads at the same relative speed. Because of the reduced drum speed, the tape head

path is at a more acute angle than in normal read mode (Figure 6). Therefore, the head can

only read a small amount of data from the track as it crosses it. To make this small piece of

data usable, sub-code data is written in small, self-contained units among the user data. With

DDS-1 and DDS-2, sub-code data is encoded on the track. However, with DDS-3, sub-code

data is in the fragment headers (areas of the tape not intended for recording) and does not use

space on the track that could be used to store data.

Tracks

Tape edge

Head motion during

fast search

Normal head motion

Figure 6: Normal and fast search tape head paths.

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ECG036/1297

DDS-3’s read-after-write (RAW) capability also improves the data transfer rate. RAW refers

to the ability of a DDS drive to read and verify data immediately after it has been written. This

is possible because of the extra set of read and write heads that DAT drives have. In the event

of an error, the drive can rewrite the incorrect data repeatedly until it is read back successfully.

This ability eliminates the need for a time-consuming validation phase.

Improved Reliability

DDS-3 technology offers improved reliability over DDS-1 and DDS-2 technology with better

error detection and correction and a more effective automatic head cleaner.

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DDS formats have three levels of error correction code (ECC): C1, C2, and C3. Additional

error detection and correction techniques, such as RAW, data randomizing, checksums, and N-

group writing, are also features of DDS-3. These techniques, combined with ECC, provide an

extremely high level of data integrity.

DDS-3 has improved C1 error detection over DDS-1 and DDS-2. DDS-3 is able to distinguish

which block in a block pair is bad and correct only the bad block. This ability decreases the

verall error rate since two C1 errors are assumed for every C1 correction with DDS-2. It also

decreases the need for C2 correction, which is used when the maximum number of C1

corrections is reached. Additionally, DDS-3 knows what correction needs to be applied to

data before it reached Error Correction Code. This ability allows for spending more time on

correction than detection.

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One of the best ways to avoid errors is to clean the tape heads regularly1. In addition to

cleaning the drive regularly with a cleaning tape, DDS-3 has an automatic head cleaner that

touches the head every five minutes of tape movement to remove debris during the write or

read recovery procedure. Automatic head cleaning also occurs when normal error correction

fails.

The DDS-3 internal head cleaner has an improved design, which provides more effective

cleaning over the latest DDS-2 designs. The new sweeper brush (Figures 7 and 8) cleans the

head better than previous foam head cleaners. This eliminates occurrences of head-to-tape

separation, reducing error rates. The automatic head cleaner enhances reliability and reduces

user maintenance requirements, reducing the frequency of cleaning by a conservative factor of

four.

Figure 7: DDS-3 head cleaner. Figure 8: DDS-3 head cleaner in DAT drive.

1For information on cleaning DAT drives, please see white paper, Tape Drives, Media, and the Importance of

Cleaning, document number ECG079.1197.

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The DDS-3 DAT drives use a 12 GB data cassette (Figure 9).

Figure 9: DAT tape cassette (actual size).

Approximately 2000 passes (or 6000 hours of tape pulling) can be made over a section of tape

in a DAT drive before performance degradation begins. A pass is defined as any forward or

reverse motion of the tape media across the read or write heads. Tape pulling is defined as

anytime the drive is reading, writing, or tensioning the tape. Depending on the backup

application and various other factors, up to six passes occur during each backup session. From

a practical standpoint, if a tape were used for a weekly backup, it should last for two years.

However, because DDS cassettes are so inexpensive, Compaq recommends replacing them

sooner to ensure a reliable backup.

Low Total Cost of Ownership

DDS-3 tape drives offer low total cost of ownership in two ways. First, DDS-3 drives are

compatible with existing DDS-1 and DDS-2 media. This compatibility provides a natural

upgrade path and investment protection for users of existing DDS format devices, reducing

total cost of ownership. Because DDS-3 was also developed with future DDS formats in mind,

the DDS-3 media are designed to be forward compatible.

Second, DAT cassettes are inherently inexpensive. With other tape storage technologies, the

cassette bodies are precision made to align the tape accurately with the heads. With DAT

technology, the precision alignment capability is built into the drive mechanism instead. This

makes DAT cassettes far less expensive than other forms of tape drive media.

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ECG036/1297

BACKUP SOFTWARE

Compaq developed the 12/24 GB DAT Drive (Figure 10) in accordance with the DDS-3

specification to provide high-performance, reliable backups for storage requirements of 24 GB

or less. Compaq performs extensive hardware and software testing to ensure a highly

integrated and compatible solution for Compaq systems.

Note: Please see the appendix for results of performance testing of the Compaq 12/24 GB

DAT Drive.

Figure 10: Compaq 12/24 GB DAT Drive being inserted into a ProLiant server.

The 12/24 GB DAT Drive is designed with TapeAlert support. TapeAlert is a tape drive status

monitoring and messaging utility that automatically detects media and drive problems that

could have a negative impact on backup quality. TapeAlert provides easy-to-understand error

messages as errors arise and recommends appropriate corrective action.

CA\Cheyenne Software

CA\Cheyenne ARCserve® for Windows NT and for Novell NetWare 3.x and IntranetWare 4.0

all support the Compaq 12/24 GB DAT Drive. ARCserve provides advanced industry features

such as:

•RAID-1 real time mirroring

•Disaster-recovery protection

•Integrated virus scanning

•RAID-5 hardware fault tolerance

ARCserve has RAID-1 mirroring and disaster-recovery protection capabilities. In addition,

ARCserve’s integrated virus scanning feature helps prevent virus infection of the backup

media. RAID-5 hardware fault tolerance enables backup and recovery to continue unabated,

even in the event of a drive failure.

In addition to ARCserve, CA\Cheyenne has JETserve Plus®backup software, which supports

the Compaq 12/24 GB DAT Drive. JETserve Plus is a server-based, ultra high-speed backup

product designed for both small, single NetWare server environments and for large NetWare

servers. JETserve allows for local backup, restore, and disaster recovery of the entire server in

short amount of time.

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Dieses Handbuch passt für folgende Modelle

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