Maximum buffer size for read from magtape?

[Steve Wall]

Yesterday I was trying to read a mag tape with a *very* large
block size (I think it was ~80k, but I'm not sure..). I don't
know why we received this tape with such a huge block size, but
that's not what concerns me.

Anyway, I tried to read the thing with the following command:

	dd if=/dev/rmt12 of=foobar conv=ascii bs=64k

and I received the following system error on the read:

	/dev/rmt12: No such device or address	(errno=6, ENXIO)

When I changed the dd to:

	dd if=/dev/rmt12 of=foobar conv=ascii bs=63k   NOTE: 63k rather than 64k

I no longer got the error message. I'm not extremely knowledgeable about
the UNIX I/O system, but this seems rather strange. Is the problem
in the I/O system itself, or does it have to do with the device I was
talking to (Kennedy 9300 drive, using tm(4) driver)? Is there a
maximum buffer size in the I/O system that I was overflowing? The
64k limit suggests something held over from the PDP version of UNIX
(We're running 4.2BSD on a 750). I have a feeling it's in the controller
or driver, since I don't have trouble reading such large chunks off
of a disk.

Anyway, I'm fairly confused and would appreciate any help from a wizard
or two. I was unable to extract all of the data from the tape because the
first (very large) block was truncated.

Thanks for the help,

Steve Wall
..!ucbvax!wall