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From: ed@mtxinu.UUCP (Ed Gould)
Newsgroups: comp.unix.questions,comp.sources.wanted
Subject: Re: Backup Utilities for Unix
Message-ID: <353@mtxinu.UUCP>
Date: Mon, 4-May-87 02:25:26 EDT
Article-I.D.: mtxinu.353
Posted: Mon May  4 02:25:26 1987
Date-Received: Tue, 5-May-87 04:54:06 EDT
References: <1308@ci-dandelion.UUCP> <4360@videovax.Tek.COM> <1422@ho95e.ATT.COM>
Reply-To: ed@mtxinu.UUCP (Ed Gould)
Organization: mt Xinu, Berkeley
Lines: 37
Keywords: system management backups dump restore
Xref: mnetor comp.unix.questions:2136 comp.sources.wanted:1082

>Incremental backups have their risks - they don't notice if files have
>been *removed*, and can get confused if links change.  But they're easy
>to use with live systems, and are normally adequate when used with
>weekly or biweekly full backups.

Real incremental dumps - like those made with dump - *do* notice that
files have been removed or that links have changed.  This has been true
since the Sixth Edition (although there were bugs in the V6 dump/restor
that were fairly serious).  The serious bugs were fixed in V7, and
facilities for easier extraction of single files from a dump were added
in 4.2BSD restore (other facilities were there all along, but they were
clumsy to use).

>On System V, you don't have dump/restor - a common approach is to use
>volcopy (a souped-up dd which can handle multiple tapes for a single
>disk slice) for image copies, and do incrementals with find/cpio.

Why USG decided to drop dump/restor from System III (or maybe it was
still distributed with SysIII but discouraged), I'll never understand.
Extracting one file from a volcopy tape is near on impossible unless
there's a spare disk partition available; many sites don't have enough
free disk to allow that.  Especially with large disks and large
partitions, it's hardly economical to keep 150Mbytes (a typical largest
partition size) free all the time.

(It actually is possible to extract one file from a volcopy of a
filesystem, but in general it can take as many as four passes over all
of the tapes:  One to read the inode and the single indirect blocks, a
second to read double indirect blocks, a third to read triple indirect
blocks and a fourth to read the actual data.  Of course, for a small
file, fewer passes may suffice.)

-- 
Ed Gould                    mt Xinu, 2560 Ninth St., Berkeley, CA  94710  USA
{ucbvax,decvax}!mtxinu!ed   +1 415 644 0146

"A man of quality is not threatened by a woman of equality."