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From: dricej@drilex.UUCP (Craig Jackson)
Newsgroups: comp.unix.wizards
Subject: Re: Backups on Live Systems
Message-ID: <203@drilex.UUCP>
Date: Sat, 6-Jun-87 09:55:41 EDT
Article-I.D.: drilex.203
Posted: Sat Jun  6 09:55:41 1987
Date-Received: Wed, 10-Jun-87 04:11:47 EDT
References: <132@dvm.UUCP> <725@aramis.rutgers.edu> <20070@sun.uucp>
Reply-To: dricej@drilex.UUCP (Craig Jackson)
Organization: Data Resources/McGraw-Hill, Lexington, MA
Lines: 50
Keywords: backup, dump, filesystem, live
Summary: Lots of places have to do it live.

Speaking of operating systems in general, and sites in general, there are
many sites that can afford to be down several hours a day for backups.
In particular, I work at a Burroughs time-sharing site, with worldwide
users.  It's nearly always prime time for somebody around the world.

In article <20070@sun.uucp> shannon@sun.uucp (Bill Shannon) writes:
>I'm curious, does anyone have any really good ideas about how to
>do reliable backups on active filesystems?  Would such backups,
>by necessity, have to be done through the filesystem, rather than
>through the raw device as dump does?

You almost have to do it through the filesystem.  It's a price you pay to stay
live.

>Also, what would you *expect* from a full dump taken on an active
>filesystem?  If you had to restore that dump, what state of the
>filesystem would you expect to restore?  What sort of guarantees
>of consistency would you want from such a dump scheme?  When the
>filesystem is inactive, it's easy to guarantee a certain consistency,
>but when it's (potentially very) active, what can you guarantee?

I expect each file in the system to be restored to the state it had when it
was copied.  If it was actively being written to at the time of the copy,
then you take your lumps.  Most aren't; they're just being read.  If you
really care, DBMS's generally have special backup procedures that handle
this.

Most systems that I've seen that do things like this do not handle
file deletions; unless you've totally lost the disk most user's don't use
that feature anyway.  It's really only an issue for incrementals.

>Does anyone know how other systems handle this problem?  What does
>VMS do?  TOPS-20?  Multics?  others?

On our Burroughs, there is no specialized dump program, nor any concept of
the raw disk (except for maintenance).  The dumps are handled by the rough
analog of 'tar' or 'cpio', from automatically constructed batch jobs.
There is a separate system for figuring out what needs to be dumped &
constructing the jobs, and for retrieving something from a dump. We wrote
our own; most sites either do that, or buy a commercial package.  The vendor
does not market a specific dump program.

>					Bill Shannon
>					Sun Microsystems, Inc.


-- 
Craig Jackson
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