As I recall, a DLT4000 has two heads, and would require 64 passes to fill
128 tracks.  The DLT7000 and DLT8000 have more heads (four, I think), and
therefore make fewer passes.

-----Original Message-----
From: Jeff Woods [mailto:[log in to unmask]]
Sent: Thursday, January 18, 2001 10:15 PM
To: [log in to unmask]
Subject: Re: DLT tape questions


At  02:42 AM 1/18/01, Paul Thompson wrote:
>Any idea what the life-span of a DLT tape is? We are coming across a
>problem restoring from couple of tapes which we have had for around 3
>years. Is there a time when the tapes should be renewed?

At  06:47 AM 1/18/01, [log in to unmask] suggests:
>Well before 3 years old!  Last time I checked, no tape manufacturer
>guarantees their product beyond 10 uses.  Obviously you can continue to
>use them past that number, & will usually be OK, but I wouldn't want to
>have any 'use once a week' tapes taht are more than 6 months old.

At  09:03 AM 1/18/01, Denys Beauchemin replied simply:
>Visit www.dlttape.com for information.

There's a lot of good info at this site even though it's (just a
little!  ;) slanted to marketing DLT.

At  09:23 AM 1/18/01, Johnson, Tracy responded:
>I went to the site, and I found something that pertains to the original
>question.  After about fifty clicks of looking for the right link I found
>the shelf-life for all forms of DLT is 30 years.

 From http://www.dlttape.com/education/whitepapers/tapepath.asp
>A low-stress tape path, contoured head, and self-cleaning design provide
>DLTtape drives with 1,000,000 tape passes per cartridge and a head life of
>30,000 hours. All DLTtape cartridges provide a shelf life in excess of 30
>years with just a five percent loss in magnetic strength.

Note that the "million passes" doesn't mean "million uses" since the tape
makes multiple passes across the head in a single "full use".  DLT stands
for "Digital [as in the company that invented it] Linear Tape" which as I
understand it writes linearly to a single track at a time and so makes a
variable number of full passes across the tape head depending on how full
the tape is.  But every time you put the tape in and do anything with it
you make at least two passes across the first part of the tape...  Once
going in and once again coming back.  That's the spot on the tape that's
almost certainly going to have the greatest number of passes in normal use.

 From the same URL:
>The DLTtape head guide assembly is one of the key components that give
>DLTtape drives an advantage over other drives on the market today. The
>DLTtape HGA has evolved to meet the ever-increasing need for higher
>capacity drives and higher data density. he six-roller DLTtape head guide
>design was first introduced in 1989. The new head guide provided the
>accuracy required to increase track density from 48 tracks to 128 racks
>across the half-inch wide tape. Today, the Quantum DLT7000 drive records
>208 tracks across the same half-inch tape.

I gather that means that a DLT7000 makes 208 passes across the head if you
fill the tape.  (I expect that the DLT8000 uses more tracks and more
passes, but a DLT4000 uses less.  I also expect that means the number of
tracks on a DLTtapeIII is the same on all the drives [since the capacity
is] and that it's probably fewer still.)  At 208 passes of the tape over
the heads that's still more than 4800 uses (full read or write passes)
across the tape before you meet the rating.

Another factor that can dramatically increase the number of head passes
would be the drive falling out of streaming mode (making the tape stop,
rewind and ramp up to speed again, often referred to as "shining the tape")
very often during access, perhaps because the drive is faster than the rest
of the system or because of retries on errors.

--
Jeff Woods