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From: dhesi@rb442.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Backups on Live Systems
Message-ID: <105@rb442.UUCP>
Date: Wed, 10-Jun-87 00:31:33 EDT
Article-I.D.: rb442.105
Posted: Wed Jun 10 00:31:33 1987
Date-Received: Sat, 13-Jun-87 04:12:43 EDT
References: <132@dvm.UUCP> <725@aramis.rutgers.edu> <20070@sun.uucp> <6279@pur-ee.UUCP> <2990@cit-vax.Caltech.Edu>
Reply-To: dhesi@rb442.UUCP (Rahul Dhesi)
Organization: CS Dept., Ball State University, Muncie, Indiana
Lines: 30

In article <2990@cit-vax.Caltech.Edu> mangler@cit-vax.Caltech.Edu (System 
Mangler) writes:
>In article <6279@pur-ee.UUCP>, davy@pur-ee.UUCP (Dave Curry) writes:
[discussing how to deal with files that change during a dump]
>> Basically, these mods
>> tell dump to ignore any inode which has changed since the dump started.
>> This has the minor disadvantage that some files (those being modified)
>> will be missing from the dump
...
>It doesn't hurt for the file to be modified while
>dump isn't looking (so long as it stays the same type); what hurts
>is when the file is modified in the middle of being dumped, something
>you don't detect and can't do anything about even if you detected it
>(other than warn the operator to start over). 

The perfect solution is not difficult, though it will probably attract
a lot of flames.

The good folks at BBN Inc. invented it many years ago.  They called
it copy-on-write.  They used it to allow self-modifying programs
to be shareable.  We can use it to dump active filesystems.  It will
need some kernel changes.

Before you begin dumping a file, you mark it copy-on-write.  During
the dump, any attempt to write to the file doesn't change the original 
but always allocates a new block.  The dump process sees only the 
original unchanged file.  The process writing to the file sees only 
the new modified file.
-- 
Rahul Dhesi         UUCP:  {ihnp4,seismo}!{iuvax,pur-ee}!bsu-cs!dhesi