Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!leah!rpi!batcomputer!cornell!uw-beaver!blake!ogccse!littlei!uunet!mcvax!ukc!cam-cl!maj
From: maj@cl.cam.ac.uk (Martyn Johnson)
Newsgroups: comp.unix.ultrix
Subject: Re:  tar and Exabyte under Ultrix
Keywords: tar r, Exabyte 8mm, Ultrix
Message-ID: <695@scaup.cl.cam.ac.uk>
Date: 30 Mar 89 10:01:09 GMT
References: <416@seila.UUCP>
Sender: news@cl.cam.ac.uk
Organization: U of Cambridge Comp Lab, UK
Lines: 15

> i spoke to the vendor, and the tech support person thought he'd
> seen a  similar problem in the past with tar and said that he'd check,
> but sort of thought that "tar r just doesn't work".

"tar r" CAN'T work on an Exabyte.  It involves reading the last block
of the archive and rewriting it.  Exabytes can't do that, because the
data blocks written in a diagonal stripe along quite a long length of
tape, whereas the erase head is vertical and erases the full width
to tape. It is therefore impossible to erase isolated data blocks.

Incidentally, this also explains why tapemarks eat up so much of the
tape capacity.

Martyn Johnson			    maj%uk.ac.cam.cl@nss.cs.ucl.ac.uk
University of Cambridge Computer Laboratory