Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 8/7/84; site ucbvax.ARPA Path: utzoo!watmath!clyde!burl!ulysses!mhuxj!ihnp4!ucbvax!wall From: wall@ucbvax.ARPA (Steve Wall) Newsgroups: net.unix-wizards Subject: Maximum buffer size for read from magtape? Message-ID: <2248@ucbvax.ARPA> Date: Fri, 28-Sep-84 16:59:25 EDT Article-I.D.: ucbvax.2248 Posted: Fri Sep 28 16:59:25 1984 Date-Received: Sat, 29-Sep-84 09:51:48 EDT Distribution: net Organization: University of California at Berkeley Lines: 35 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