The major tape media are the 4mm, 8mm, QIC, mini-cartridge and DLT.
4mm tapes are replacing QIC as the workstation backup media of choice. This trend
accelerated greatly when Conner purchased Archive, a leading manufacturer of QIC drives,
and then stopped production of QIC drives. 4mm drives are small and quiet but do not have
the reputation for reliability that is enjoyed by 8mm drives. The cartridges are less
expensive and smaller (3 x 2 x 0.5 inches, 76 x 51 x 12 mm) than 8mm cartridges. 4mm,
like 8mm, has comparatively short head life for the same reason, both use helical
scan.
Data throughput on these drives starts ~150 kB/s, peaking at ~500 kB/s. Data
capacity starts at 1.3 GB and ends at 2.0 GB. Hardware compression, available
with most of these drives, approximately doubles the capacity. Multi-drive tape library
units can have 6 drives in a single cabinet with automatic tape changing. Library
capacities reach 240 GB.
The DDS-3 standard now supports tape capacities up to 12 GB (or 24 GB
compressed).
4mm drives, like 8mm drives, use helical-scan. All the benefits and drawbacks of
helical-scan apply to both 4mm and 8mm drives.
Tapes should be retired from use after 2,000 passes or 100 full backups.
8mm tapes are the most common SCSI tape drives; they are the best choice of exchanging
tapes. Nearly every site has an Exabyte 2 GB 8mm tape drive. 8mm drives are
reliable, convenient and quiet. Cartridges are inexpensive and small (4.8 x 3.3 x 0.6
inches; 122 x 84 x 15 mm). One downside of 8mm tape is relatively short head and tape
life due to the high rate of relative motion of the tape across the heads.
Data throughput ranges from ~250 kB/s to ~500 kB/s. Data sizes start at
300 MB and go up to 7 GB. Hardware compression, available with most of these
drives, approximately doubles the capacity. These drives are available as single units or
multi-drive tape libraries with 6 drives and 120 tapes in a single cabinet. Tapes are
changed automatically by the unit. Library capacities reach 840+ GB.
The Exabyte “Mammoth” model supports 12 GB on one tape (24 GB
with compression) and costs approximately twice as much as conventional tape drives.
Data is recorded onto the tape using helical-scan, the heads are positioned at an
angle to the media (approximately 6 degrees). The tape wraps around 270 degrees of the
spool that holds the heads. The spool spins while the tape slides over the spool. The
result is a high density of data and closely packed tracks that angle across the tape
from one edge to the other.
QIC-150 tapes and drives are, perhaps, the most common tape drive and media around.
QIC tape drives are the least expensive “serious” backup drives. The downside
is the cost of media. QIC tapes are expensive compared to 8mm or 4mm tapes, up to 5 times
the price per GB data storage. But, if your needs can be satisfied with a half-dozen
tapes, QIC may be the correct choice. QIC is the most common tape drive. Every site has a QIC drive of some
density or another. Therein lies the rub, QIC has a large number of densities on
physically similar (sometimes identical) tapes. QIC drives are not quiet. These drives
audibly seek before they begin to record data and are clearly audible whenever reading,
writing or seeking. QIC tapes measure 6 x 4 x 0.7 inches (152 x 102 x
17 mm).
Data throughput ranges from ~150 kB/s to ~500 kB/s. Data capacity ranges
from 40 MB to 15 GB. Hardware compression is available on many of the newer QIC
drives. QIC drives are less frequently installed; they are being supplanted by DAT
drives.
Data is recorded onto the tape in tracks. The tracks run along the long axis of the
tape media from one end to the other. The number of tracks, and therefore the width of a
track, varies with the tape's capacity. Most if not all newer drives provide
backward-compatibility at least for reading (but often also for writing). QIC has a good
reputation regarding the safety of the data (the mechanics are simpler and more robust
than for helical scan drives).
Tapes should be retired from use after 5,000 backups.
DLT has the fastest data transfer rate of all the drive types listed here. The 1/2"
(12.5mm) tape is contained in a single spool cartridge (4 x 4 x 1 inches; 100 x 100 x 25
mm). The cartridge has a swinging gate along one entire side of the cartridge. The drive
mechanism opens this gate to extract the tape leader. The tape leader has an oval hole in
it which the drive uses to “hook” the tape. The take-up spool is located
inside the tape drive. All the other tape cartridges listed here (9 track tapes are the
only exception) have both the supply and take-up spools located inside the tape cartridge
itself.
Data throughput is approximately 1.5 MB/s, three times the throughput of 4mm,
8mm, or QIC tape drives. Data capacities range from 10 GB to 20 GB for a single
drive. Drives are available in both multi-tape changers and multi-tape, multi-drive tape
libraries containing from 5 to 900 tapes over 1 to 20 drives, providing from 50 GB
to 9 TB of storage.
With compression, DLT Type IV format supports up to 70 GB capacity.
Data is recorded onto the tape in tracks parallel to the direction of travel (just
like QIC tapes). Two tracks are written at once. Read/write head lifetimes are relatively
long; once the tape stops moving, there is no relative motion between the heads and the
tape.
AIT is a new format from Sony, and can hold up to 50 GB (with compression) per
tape. The tapes contain memory chips which retain an index of the tape's contents. This
index can be rapidly read by the tape drive to determine the position of files on the
tape, instead of the several minutes that would be required for other tapes. Software
such as SAMS:Alexandria can operate forty or more AIT tape
libraries, communicating directly with the tape's memory chip to display the contents on
screen, determine what files were backed up to which tape, locate the correct tape, load
it, and restore the data from the tape.
Libraries like this cost in the region of $20,000, pricing them a little out of the
hobbyist market.
The first time that you try to read or write a new, completely blank tape, the
operation will fail. The console messages should be similar to:
sa0(ncr1:4:0): NOT READY asc:4,1
sa0(ncr1:4:0): Logical unit is in process of becoming ready
The tape does not contain an Identifier Block (block number 0). All QIC tape drives
since the adoption of QIC-525 standard write an Identifier Block to the tape. There are
two solutions:
-
mt fsf 1 causes the tape drive to write an Identifier Block
to the tape.
-
Use the front panel button to eject the tape.
Re-insert the tape and dump data to the tape.
dump will report “DUMP: End of
tape detected” and the console will show: “HARDWARE FAILURE info:280 asc:80,96”.
rewind the tape using: mt rewind.
Subsequent tape operations are successful.
