Compaq 108164-003 - ProLiant - 800 User manual

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ECG036/1297
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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
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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.
©1997 Compaq Computer Corporation. All rights reserved. Printed in the U.S.A.
DDS-3: Enhancing DAT Reliability, Capacity, and Performance
First Edition (December 1997)
Document Number ECG036/1297

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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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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
Read head
Write head

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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
,QFUHDVHG /LQHDU %LW 'HQVLW\
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.
,QFUHDVHG )RUPDW (IILFLHQF\
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.
,QFUHDVHG 8VH RI 7UDFN 6SDFH
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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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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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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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.
(UURU 'HWHFWLRQ DQG &RUUHFWLRQ
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.
'ULYH &OHDQLQJ
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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'$7 0HGLD 'XUDELOLW\
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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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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Seagate Software
Seagate Backup Exec®for Microsoft Windows NT, Novell NetWare 3.x and IntranetWare 4.0
all support the Compaq 12/24 GB DAT Drive. The Seagate software complements the 12/24
GB DAT Drive with advanced industry features, such as:
•Intelligent disaster recovery
•Dynamic drive pooling in the Windows NT environment
•Fault tolerant backup
•Media mirroring which provides RAID (redundant array of independent disks) Level
1 (RAID-1) in the Windows NT environment
•Agent accelerator technology
These software features protect customers’ online data in most cases. Intelligent disaster
recovery provides the only diskette-based, “point-in-time” recovery option that eliminates the
need to re-load the operating system first. Drive pooling and fault tolerant backup features
ensure that backups occur consistently by using all available DDS-3 resources. With RAID-1
functionality, customers can mirror backups (one copy for onsite and one for offsite storage).
Backup Exec’s intuitive user interface provides an easy-to-use approach to backup and restore,
and agent accelerator technology, which maximizes backup speeds, greatly enhances backup
performance.
CONCLUSION
DDS-3 is a cost effective, reliable, high-performance backup solution for mid-size companies
or branch offices that have storage requirements of 24 GB or less and are running Windows
NT or Novell NetWare3.x or IntranetWare 4.0. The new DDS-3 technology has several
significant improvements over existing DDS-2 technology, including increased capacity, faster
transfer rate, and improved reliability and offers low total cost of ownership. Compaq’s 12/24
BG DAT Drive provides a highly integrated backup solution and is supported by the Seagate
Backup Exec, CA\Cheyenne ARCserve, and CA\Cheyenne JETserve data management
software. The 12/24 GB DAT Drive demonstrates Compaq’s commitment to providing viable
business solutions for all levels of enterprise computing.

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APPENDIX: Performance Test Results
Test 1: Local backup under Microsoft Windows NT 4.0 with
Cheyenne ARCserve backup software
Hardware ProLiant 6000 with three Pentium Pro 200MHz
processors, 512 K cache, 320 MB memory, one
SMART-2 Array Controller, fourteen 4.3 GB ultra-wide
SCSI drives
Operating system Microsoft Windows NT 4.0
Backup software Cheyenne ARCserve 6.0 (Build 516)
Backup drive Compaq 12/24 GB DAT
DAT drive firmware Version 2.86
Data set per drive 512 MB of 1 MB files
RAIT level RAIT-0
Results:
Compression
Ratio RAIT Setup Backup Speed (MB/min) Restore Speed (MB/min) Image or File- by-File (FBF)
1:1 1:1 48.31 48.38 FBF
2:1 1:1 91.26 90.99 FBF
4:1 1:1 132.01 132.01 FBF
1:1 1:1 48.71 43.65 Image
2:1 1:1 90.79 45.66 Image
4:1 1:1 130.65 65.05 Image
2:1 2:1 90.72 90.45 FBF
4:1 2:1 254.12 135.53 FBF
2:1 2:1 90.00 45.26 Image
4:1 2:1 239.85 68.52 Image
2:1 2:2 183.62 109.65 FBF
4:1 2:2 224.22 245.92 FBF
2:1 2:2 168.62 92.43 Image
4:1 2:1 237.99 116.29 Image
2:1 4:1 176.20 178.26 FBF
4:1 4:1 260.64 222.59 FBF
2:1 4:2 162.38 86.69 Image
4:1 4:2 232.61 115.43 Image

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Test 2: Local backup under Novell IntranetWare SMP with
Cheyenne ARCserve backup software
Hardware ProLiant 6000 with three Pentium Pro 200MHz
processors, 512 K cache, 320 MB memory, one
SMART-2 Array Controller, fourteen 4.3 GB ultra-wide
SCSI drives
Operating system Novell IntranetWare SMP
Backup software Cheyenne ARCserve 6.1
Backup drive Compaq 12/24 GB DAT
DAT drive firmware Version 2.86
Data set per drive 512 MB of 1 MB files
RAIT level RAIT-0
Results:
Compression
Ratio RAIT Setup Backup Speed (MB/min) Restore Speed (MB/min) Image or File- by-File (FBF)
1:1 1:1 51.62 51.86 FBF
2:1 1:1 92.21 91.28 FBF
4:1 1:1 103.14 102.62 FBF
2:1 3:1 142.84 125.23 FBF
2:1 3:2 148.23 135.25 FBF
2:1 5:1 148.23 140.23 FBF
Test 3: Local backup under Microsoft Windows NT with
Seagate Backup Exec software
Hardware ProLiant 6000 with three Pentium Pro 200MHz
processors, 512 K cache, 320 MB memory, one
SMART-2 Array Controller, fourteen 4.3 GB ultra-wide
SCSI drives
Operating system Microsoft Windows NT 4.0
Backup software Seagate Backup Exec version 7.0 Rev. 554a
Backup drive Compaq 12/24 GB DAT
DAT drive firmware Version 2.86
Data set per drive 512 MB of 1 MB files
RAIT level RAIT-0
Results:
Compression
Ratio RAIT Setup Backup Speed (MB/min) Restore Speed (MB/min) Image or File- by-File (FBF)
1:1 1:1 50.37 50.79 FBF
2:1 1:1 89.10 91.52 FBF
4:1 1:1 127.17 132.15 FBF
2:1-32k block 1:1 90.70 FBF

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Test 4: Local backup under Novell IntranetWare SMP with
Seagate Backup Exec software
Hardware ProLiant 6000 with three Pentium Pro 200MHz
processors, 512 K cache, 320 MB memory, one
SMART-2 Array Controller, fourteen 4.3 GB ultra-wide
SCSI drives
Operating system Novell IntranetWare SMP
Backup software Seagate Backup Exec version 7.0 Rev. 554a
Backup drive Compaq 12/24 GB DAT
DAT drive firmware Version 2.86
Data set per drive 512 MB of 1 MB files
RAIT level RAIT-0
Results:
Compression
Ratio RAIT Setup Backup Speed (MB/min) Restore Speed (MB/min) Image or File- by-File (FBF)
1:1 1:1 50.40 50.40 FBF
2:1 1:1 90.60 91.00 FBF
4:1 1:1 129.30 97.60 FBF
Test 5: Local backup under Novell IntranetWare SMP with
Cheyenne JETserve Plus backup software
Hardware ProLiant 6000 with three Pentium Pro 200MHz
processors, 512 K cache, 320 MB memory, one
SMART-2 Array Controller, fourteen 4.3 GB ultra-wide
SCSI drives
Operating system Novell IntranetWare SMP
Backup software Cheyenne JETserve Plus version 4.0
Backup drive Compaq 12/24 GB DAT
DAT drive firmware Version 2.86
Data set per drive 512 MB of 1 MB files
RAIT level RAIT-0
Results:
Compression
Ratio RAIT Setup Backup Speed (MB/min) Restore Speed (MB/min) Image or File- by-File (FBF)
2:1 1:1 91.29 91.09 Image
2:1 2:1 178.60 - Image
2:1 3:1 270.66 - Image
2:1 4:1 352.09 - Image
2:1 2:2 182.31 - Image
2:1 4:2 180.19 - Image
2:1 8:2 361.41 - Image