What is the 'real' tape length

[David J. Camp]

Spots,

This looks pretty authoritative, so I thought you would be interested.
When I switched from /dev/rst0 to /dev/rst8 I did an experiment as
suggested in my original inquiry, and the tape length has not changed.  If
anything it is a little shorter, around 900 feet for /dev/rst8 compared to
1000 feet for /dev/rst0.  I have not figured out if I am actually getting
more data on the tape though.  -David-

Bitnet:   david at wubios.wustl               Mr. David J. Camp
Internet: david%wubios at wucs1.wustl.edu     Box 8067, Biostatistics
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Washington University Medical School       Saint Louis, MO 63110
__________

swagman at Sun.COM (Patrick J. McEvoy (Sun Entry Systems ISV Liason)):
>	The length of tape expected by dump is the real length of the
>	tape times the number of tracks minus some factor for
>	inter-record gaps.  If you use /dev/rst8 you get 9 tracks, but
>	if you use /dev/rst0 you get only 4 tracks.

No.  rst8 & rst0 spcify QUIC24 or QUIC11 format.  There is some density
difference, but mostly it is just that QUIC24 is more robust.

The 9 or 4 trackness is completely dependent upon your tape drive. The
9-track drives first use the same 4 tracks as the 4-track drives, then
they interleave 5 more tracks.  So uo to 20 Mbytes, the tapes can be used
on either kind of drive.

The confusion about tape lengths is that dump expects info in terms of 1/2
tapes that have 9 vertical bits -- so a 1600 bits-per-inch tape (linearly)
actually gives you 1600 *bytes* per inch.

Dump then figures out how much stuff to put on the tape.  If it succeeds,
it goes on to the next tape even if there is room left.  If it hits an EOT
(end of tape) it blows up.

Cartridge tapes are 450 or 600 feet times 4 or 9 tracks long.  But then
you have to divide by 9 (byte with parity) because the tracks are only 1
bit wide.

If you have a 450, just use the "c" option on dump.  If you have a 600,
you have to play with the numbers.  But remember dump thinks you are
telling it about a single-pass, 9-bit-wide tape.