Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!elroy!cit-vax!mangler
From: mangler@cit-vax.Caltech.Edu (Don Speck)
Newsgroups: comp.unix.wizards
Subject: Re: another question about dump & restore
Summary: dumping active filesystems
Keywords: dump restore file system
Message-ID: <6888@cit-vax.Caltech.Edu>
Date: 10 Jun 88 01:08:18 GMT
References: <5757@umn-cs.cs.umn.edu> <3339@phri.UUCP>
Organization: California Institute of Technology
Lines: 28

In article <3339@phri.UUCP>, roy@phri.UUCP (Roy Smith) writes:
>			  I believe there have been some mods to the 4.2BSD
> dump posted (probably by Don Speck, and probably included in the 4.3
> version of dump) which help make dump more immune to file system activity.

Those mods pre-date the parts that I did.  I first saw them in the
"mass-driver" diffs that Chris Torek sent me in January 1985.  They
skip inodes that have been zeroed (deleted) during the dump, as
opposed to the Purdue mods, which also skip inodes whose st_mtime
has changed (such as /dev/tty and /dev/rmt8).

To see how much difference the 4.3 mods made, I copied a small filesystem
to /usr/tmp, then started a simultaneous dump to tape and "rm -r".
Restore would dump core after typically 20% of each tape made by 4.2 dump,
but typically made it through 80% (and sometimes all) of each tape made
by 4.3 dump.  Part of the difference is that 4.3 dump finished before the
"rm -r", so the filesystem was not modified as much during the dump.

An "active filesystem" is one suffering deletions, renamings, and (to
a smaller extent) truncations.	Dump will not see files created during
the dump, and restore should not care about access times being updated.
Directories changing between passes II and III is what confuses restore.

By way of illustration, I once modified dump to start on the mapping
passes while the first tape was being mounted.	Restore choked on dumps
started right before the operator's meal break, which was putting a
one-hour wait between passes II and III.

Don Speck   speck@vlsi.caltech.edu  {amdahl,ames!elroy}!cit-vax!speck