What is the 'real' tape length

[nielsen at saturn.ecs.umass.edu]

> We have a Sun 3/260 running SunOS 4.0.  The tapes that I buy for the Sun
> say on them "600'", which I assume means 600 feet.  However, when I use
> dump, and specify a size longer than 600', it does not fail until
> somewhere near 1000' (at least that is what it reports).

It is tough to comment on this without seeing the entire command you used.
Later in this posting is some general info on 'dump', some of this info is
condensed from Sun Spots postings on Jun. 30 1988 and Aug 18 1988.

We shared a similar experience with tape lengths being strange.  It was
*due* to our using the a density of 10,000 (YES TEN-THOUSAND), the
standard blocking factor, and NOT using the 'c' (for cartidge) option.
Through trial and error we found that 1100 feet was a safe value.  We used
the 10,000 bpi density as was given on the tape.  The man on dump given on
the tape.  The man on dump is written primarily for reel tapes, and since
they write on 9 tracks serially (as opposed to the cartridge's 9 tracks
sequentially), we were confused between bits per inch and bytes per inch.
{A reel tape writes a byte at a time serially} Our backups AND restores
were always successful... until our tape drive died.  Before we determined
the drive was broken, we started investigating our dump parameters more
carefully and became aware that our parameters were unusual.

->	A question is: Why does density effect the tape length? 
	It *shouldn't*, but it does.

->	Another question is: Is it safe to backup using the {unusual}
command:
% /etc/dump "$level"usdf 1100 10000 /dev/rst8 "$filesystem"
	IT HAS ADVANTAGES if it is safe.  It had worked reliably for
	over a year, takes LESS THAN HALF the time, and stores
	about 60 Megs of data per tape.  I asked SUN about this, but
	they never got back to me.

Most of the following info is condensed/excerpted from Sun Spots postings
on Jun. 30 1988 [Dan Franklin] and  Aug 18 1988 [John Gilmore]:

A suggested dump command is:

/etc/dump "$level"ucsbf 4800 126 /dev/rst8 "$filesystem"
	The "c" indicates the use of a tape cartiridge
	u is to update the '/etc/dumpdates' file
	4800 is the tape length used for 600' tapes
	126 is the blocking factor
	with the c the density defaults to 1000

Lowering the blocking factor decreases the amount of data stored on
each tape {bad} and the time required for a dump {good}.

The cartridge tape drives write 9 tracks sequentially, that 
is why the 4800' tape length is used (9 X tape-length X 0.95 {safety
 factor}).  

[excerpted:

<There are two variants of the bit format that controllers record on the
<tape.  One is called QIC-11, the other is QIC-24.  QIC-11 is the original
<Archive format (Archive Corp. started the whole 1/4" streaming cartridge
<business).  When a standards committee got a hold of it, they changed it
<(of course) to QIC-24.  In both cases, the tape contains 512-byte blocks
<of data with small headers on them.  For QIC-11, the block number in the
<header is 8 bits; for QIC-24, the number is 24 bits.  That is essentially
<the only difference between the two.  It was changed because in the 
<controller could lose track of where it was on the tape.  Both formats
<hold the same amount of data on a given tape.
<
<  .... you have to specify in software [...] whether you want QIC-11 or
<QIC-24 formatting.  /dev/rst0 is QIC-11 and /dev/rst8 is QIC-24.
 ....]

Eric Nielsen
Mechanical Engineering Dept.
University of Massachusetts at Amherst