Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!rex!ukma!acp
From: acp@ms.uky.edu (ACPNET consultant)
Newsgroups: comp.unix.i386
Subject: Compressing to a tape drive
Message-ID: <14496@s.ms.uky.edu>
Date: 8 Mar 90 18:59:45 GMT
Reply-To: acp@ms.uky.edu (ACPNET consultant)
Distribution: na
Organization: U of Kentucky, Mathematical Sciences
Lines: 29

This is on an AT&T 6386 WGS running Sysv/386 3.2.1.

The hard disk in the machine is ~145 megabytes, and the tape drive only
holds 60meg, so I'm trying to fit a full disk backup on one tape with
something like

find . -print | cpio -oc | compress | dd [blocking options] > /dev/rmt/c0s0

Now, including dd in the pipe is *loads* faster than not including
it, but I haven't been able to find any dd options which let the 
tape stream.  It keeps stopping and backing up.  When using something
like ibs=32k obs=512 (the tape writes 512-byte records), dd reports
no full input blocks, all partial blocks--I assume this corresponds to
bursts produced by compress.  Something like bs=512 produces all full
blocks but doesn't affect the tape speed.

I assume something as slow as a tape drive could keep up with compress
if the buffering was done properly; it seems that when dd has a large
input buffer, it should hold off writing until the buffer was full;
this would at least allow the tape to stream for several records while
dd fills up its input buffer again.

Should I be using something other than dd?  Any other suggestions?

Kenneth Herron
-- 
acp@ms.uky.edu        University of Kentucky         ACP Network Consultant
ukma!acp         Dept. of Mathematics, room 715 POT          (606) 257-2975
                       Lexington, KY 40506