Peter Farrow wrote:
>>You might verify a tape and send it offsite only to find after a
disaster that another drive would not read it
This is a very real problem, tape drives with larger drums and wider
tapes will suffer less wear and friction on the tape, and to that end
will suffer less from azimuth problems due to age.
DLT tapes or LTO tape drives will provide better reliability over 4mm
and 8mm formats in this area, However modern tape drives, LTO and DLT
included have auto azimuth feature in them to overcome these issue.
LTO and DLT are fundamentally different than 4mm/8mm formats...
the 4mm/8mm formats are both 'helical scan', there's a pair of single
heads on a drum thats spinning very rapidly at an angle to the tape, as
the tape moves past the drum, the head describes a diagonal path on the
tape. these heads are typically spinning at 1800 rpm or 30
turns/second. head wear is a huge problem, as is the possibility of
extensive media damage from a stray particle due to the high speed of
these spinning heads. The tape is quite long and rather thin, much like
a video tape (in fact, 8mm formats like AIT are based on 8mm video
tape, and 4mm DDS formats are based on R-DAT audio tape). To implement
this, the tape path is long and complicated (ever take a VHS VCR apart
to clean the rollers? its the same thing only much smaller)
LTO and DLT are linear formats, utilizing 'serpentine' travel. LTO,
for example, the 1/2" wide tape moves at a linear 5 or so meters/second
or so, the head is stationary, and has several 100 tracks on it. the
head is mounted on a servo that follows servo tracks embedded on the
tape. the tape has as many as 700 tracks (LTO3). the tape is run
past the head for several passes to read or write the whole thing, with
one set of ~100 tracks read as the tape leaves the cartridge, then the
head repositions and reads the next set of tracks as the tape is rewound
into the cartridge, repeat this a few times. The tape is thick, sturdy,
and fairly short. The tape path is quite simple.
DAT tapes should be discarded after a dozen or so uses, while LTO tapes
are rated at nearly 300 uses, and 5000 load cycles. helical scan
heads wear due to the very high linear speed, LTO drives are rated at
250000 hours MTBF at 100% duty cycle. the linear formats are
enterprise grade
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