How do you calculate the tape length for 150MB in SunOS 3.x

[Jarkko Lavinen]

I've been trying to do a remote dump from older 3/50 (SunOS 3.5) to a
SparcStation 1+. I've had no problem at all with doing local dumps on
Sparcstation (SunOS 4.1) like : (tape device is 150MB, 18 tracks, density
1200 BPI)

/etc/dump 0ucsdtbf 590 1200 18 126 /dev/rst0 /dev/sd0g

No problem at all. The parameters are caught from the manual.

Then I tried to do a remote dump from the 3/50. The older dumps doesn't
know what a track is, and it thinks the tape as a one-track device. Thus
the length (or density) has to recalculted to compensate.  The man page
says (length*tracks)*0.9. So I made the calculation and tried this

/etc/rdump 0nucsdbf 9700 1200 126 remotehost:/dev/rst0 /dev/sd1c

Well, this causes write error after about hour of dumping. The tape simply
ends. It seems dump can't calculate it's tape usage properly. One of the
reason I've been think is record gap. Dump probably expects record gap to
be 1/18 of the real value, because the same gap is probably at the same
time and place on all tracks.

I tried to reduce the length by 10%, but it didn't help. After that I
reduced some >50% and dump went ok. I could probably estimate the correct
tape length by some kind of binary search, but it doesn't quite feel
elegant way to do it.

Can anybody out there say how the correct tape length is calculated? I
would be very thankful for comments on this.

	Jarkko Lavinen