scsi tape quirks?

[Ron Stanonik]

We recently attached an exabyte to a sun4/280 running sunos4.0.1.  While
becoming familiar with the exabyte, we ran into some quirks of scsi tape
handling; ie, the archive tape drive behaves the same.

1) The non-rewinding device seems to rewind if you close before
reading the filemark.  For example, make a tape contain one file,
two blocks long
dd if=/etc/termcap of=/dev/rst0 count=2
Now try reading the tape one block, at a time
dd if=/dev/nrst0 count=1
dd if=/dev/nrst0 count=1
Both dd's returned the same (first) block.

2) You cannot read data past a filemark, unless you first read the
filemark and close.  For example, make a tape containing two files
dd if=/etc/termcap of=/dev/nrst0 count=1
dd if=/etc/format.dat of=/dev/nrst0 count=1
Now read several blocks without closing.  For this you need a little
program that opens /dev/nrst0 and does several read's.  The first
read returns the termcap block, but successive reads only return 0.
On the other hand
dd if=/dev/nrst0
dd if=/dev/nrst0
will read first the termcap, then the format.dat.

Maybe this all is according to scsi tape semantics (if there is such a
time)?  I'm not familiar enough with scsi to say whether this is correct
or not.

1/2" drives on our vax (4.3bsd) and suns (xt, not scsi) behave
differently.  That is, the non-rewinding device doesn't rewind, and reads
will read past a filemark.  That's what I would expect.

Ron Stanonik
stanonik at nprdc.navy.mil