Helical-scan technology used on 4-mm DAT drives records large amounts of data on very-slow-moving tape. Magnetic read/write heads are mounted on a rotating drum, with an axis of rotation at 6 degrees from the perpendicular (see the figure ``
Helical Scan
''). DAT drives have two heads for reading and two for writing. The tape wraps 90 degrees around the drum's circumference, and the heads move in a spiral motion from the bottom to the top of the tape. The drum rotates at 2000 rpm, and the 4-mm tape moves slowly in the same direction at 8.15 millimeters per second (or 0.32 inch per second). The diametrically opposed heads describe portions of a helix on the tape.
Each track is written diagonally from top to bottom. The heads are wider than the written tra
cks, so each new track overlaps the previous one, wasting no tape between tracks. Overlapping tracks would normally result in cross talk between adjacent tracks when reading data from the tape, but the device minimizes cross talk by angling the heads 20 degrees relative to the data track (the azimuth angle) and in opposite directions from each other.
When data is read from the tape, the read head receives a much stronger signal from data written to the same azimuth angle. Angling permits very close packing of tracks and very high data densities. ATF (Automatic Track Finding) circuitry keeps the head centered on the track by balancing the weaker signals from adjacent (i.e., off-azimuth) tracks.
In the early days of DAT's evolution, DAT devices used one of two proposed low-level formatting standards: DDS, developed jointly by HP and Sony, and Data/DAT, developed by Hitachi. DDS offers slightly more storage capacity and faster sustained transfer rates than Data/DAT. DDS devices write data sequentia
lly, appending data to the existing information, and they can read data randomly, beginning at any point. Data/DAT drives can overwrite existing data files in place, reducing the inefficiencies of multiple copies of the same file. However, the market appears to have standardized on DDS and a few extensions of DDS.
DDS: The DDS format represents a modification of the DAT technology. Unlike the continuous data stream that's produced by the DAT format, DDS constructs a sequence of fixed-capacity groups on the tape. DDS packs up to 2 GB on a 60-, 90-, or 120-meter tape running at the same speed as DAT. The best-case scenario for a DAT recording is a sustained transfer rate of 183 Kbps to fill a tape during 2 hours of transfer time.
DDS-2: This was developed to allow data transfer to occur in SCSI-2 burst mode. DDS-2 doubles the density of DDS, but it maintains full DDS functionality and ensures backward compatibility.
DDS-DC: Similar to DDS-2, DDS-DC was established to include a data-compress
ion standard. It allows uncompressed data to be stored in a way that maintains full DDS functionality and ensures backward compatibility with existing DDS drives.
The types of drive mechanisms in the DAT drives we tested differ widely; they are sourced by HP, Conner, WangDAT/Rexon, Sony, and Exabyte. In general, DAT drives based on HP's C1533A drive mechanism provide superior performance when compared to DAT drives from the other major drive manufacturers. In fact, the HP JetStore 6000e's NT performance is so fast that it ties for third place with TTI's CTS-2110, which is based on DEC's DLT2000 drive mechanism.
Conner-manufactured drive mechanisms also perform well. The Conner MS8000DAT achieves its best performance when tested in a server-attached configuration using the vendor-supplied 16-bit AT SCSI adapter, even surpassing the performance of drives using 32-bit bus-mastering SCSI controllers. Using a specialized hardware interface with an optimized device driver appears to be the major facto
r in the excellent performance of the MS8000DAT in the server-attached configuration.
THE BEST LAN SOLUTION
SERVER-ATTACHED
Conner Storage Systems MS8000DAT
Based on Conner's Python 28388 drive mechanism, the MS8000DAT garnered the top performance scores in our server-attached category. This drive is a great choice for backing up and archiving data in a NetWare environment. If you're economy-minded, the GigaTrend Turbo II costs $1204 less than the MS8000DAT, but it doesn't lag far behind in performance.
Drive Mech./Mfr Price
BEST Conner MS8000DAT Python 28388/Conner $2650
RUNNER-UP GigaTrend Turbo II C1533A/HP $1975
Overall
Drive Mech./Mfr Server Workstation NT
BEST Conner MS8000DAT Python 28388/Conner 9.54 N/A 7.52
RUNNER-UP GigaTrend Turbo II C1533A/HP 9.40 N/A
7.60
Performance
Drive Mech./Mfr Server Workstation NT
BEST Conner MS8000DAT Python 28388/Conner 10.00 N/A 7.31
RUNNER-UP GigaTrend Turbo II C1533A/HP 9.61 N/A 7.21
Drive Mech./Mfr Features Usability
BEST Conner MS8000DAT Python 28388/Conner *** ***
RUNNER-UP GigaTrend Turbo II C1533A/HP **** ***
KEY
Excellent ****
Good ***
Fair **
Poor *
FOR LOCAL BACKUP...
WORKSTATION-ATTACHED
Storage Dimensions TDB-8005
The top three drives for local backup employ HP's C1533A drive mechanism. These OEM tape drives--Storage Dimensions' TDB-8005, Optima Technology's MiniPak F8000DAT, and MicroNet Technology's SS-D16000/EISA--all excel at local backup, but the more expensive TDB-8005 takes the crown. Storage Dimensions' drive and
the MiniPak F8000DAT both feature 4-to-1 (16 GB to 4 GB) compression. The SS-D16000/EISA supports 16 GB of data and has a 2-to-1 compression scheme.
Drive Mech./Mfr Price
BEST Storage Dimensions TDB-8005 C1533A/HP $2315
RUNNER-UP Optima MiniPak F8000DAT C1533A/HP $1879
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP $2585
Overall
Drive Mech./Mfr Server Workstation NT
BEST Storage Dimensions TDB-8005 C1533A/HP N/A 9.31 8.18
RUNNER-UP Optima MiniPak F8000DAT C1533A/HP N/A 8.97 8.68
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP N/A 8.74 8.68
Performance
Drive Mech./Mfr Server Workstation NT
BEST Storage Dimensions TDB-8005 C1533A/HP N/A 9.4
5 7.95
RUNNER-UP Optima MiniPak F8000DAT C1533A/HP N/A 9.26 8.87
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP N/A 8.69 8.61
Drive Mech./Mfr Features Usability
BEST Storage Dimensions TDB-8005 C1533A/HP **** ***
RUNNER-UP Optima MiniPak F8000DAT C1533A/HP **** ***
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP **** ***
KEY
Excellent ****
Good ***
Fair **
Poor *
OUTSTANDING NT PERFORMANCE
WINDOWS NT
Hewlett-Packard HP JetStore 6000e
The HP JetStore 6000e is second to none when it comes to backing up and restoring files on Windows NT. It is a speedy backup solution that is bundled with Cheyenne Software's ARCserve 5.1 backup software, as well as data-recovery utilities and the JetSafe set of diagnostic tools.
Drive Mech./Mfr Price
BEST HP JetSto
re 6000e C1533A/HP $3714
RUNNER-UP Ariel-DAT ADE-8500 C1533A/HP $2399
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP $1828
Overall
Drive Mech./Mfr Server Workstation NT
BEST HP JetStore 6000e C1533A/HP 8.41 N/A 9.19
RUNNER-UP Ariel-DAT ADE-8500 C1533A/HP 8.85 N/A 8.73
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP N/A 8.74 8.68
Performance
Drive Mech./Mfr Server Workstation NT
BEST HP JetStore 6000e C1533A/HP 8.29 N/A 9.34
RUNNER-UP Ariel-DAT ADE-8500 C1533A/HP 8.86 N/A 8.71
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP N/A 8.69 8.61
Drive Mech./Mfr Features Usability
BEST HP JetStore 6000e C1533A/HP
**** ***
RUNNER-UP Ariel-DAT ADE-8500 C1533A/HP **** ***
RUNNER-UP MicroNet SS-D16000/EISA C1533A/HP **** ***
KEY
Excellent ****
Good ***
Fair **
Poor *
RAW-SPEED LEADER
PERFORMANCE
Hewlett-Packard HP JetStore 6000e
The NT/HP marriage worked again when we tested tape drives solely on their raw performance while backing up on Windows NT. Five of the top six drives are originally manufactured by HP. The only non-HP NT backup device that breaks into the winner's circle is IBM PC Co.'s IBM 4/10 GB 4-mm drive, the first runner-up to the HP JetStore 6000e.
Drive Mech./Mfr Price
BEST HP JetStore 6000e C1533A/HP $3714
RUNNER-UP IBM 4/10 GB 4-mm 4326NP/RP/Conner $1699
Overall
Drive Mech./Mfr Server Workstation NT
BEST HP JetStore
6000e C1533A/HP 8.41 N/A 9.19
RUNNER-UP IBM 4/10 GB 4-mm 4326NP/RP/Conner N/A 8.31 8.55
Performance
Drive Mech./Mfr Server Workstation NT
BEST HP JetStore 6000e C1533A/HP 8.29 N/A 9.34
RUNNER-UP IBM 4/10 GB 4-mm 4326NP/RP/Conner N/A 8.61 8.93
Features Usability
BEST HP JetStore 6000e C1533A/HP **** ***
RUNNER-UP IBM 4/10 GB 4-mm 4326NP/RP/Conner **** *
KEY
Excellent ****
Good ***
Fair **
Poor *
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With helical-scan technology, each new track overlaps the previous one, wasting no tape between tracks.
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The Technology Behind 4-MM DAT Drives
