Dump in multi-user mode?

[David Collier-Brown]

> In article <362 at ntvax.UUCP> chris at ntvax.UUCP (Chris Britton) writes:
>> What are the consequences of running "dump" in multi-user mode? 

>From article <352 at lilink.UUCP>, by mikej at lilink.UUCP (Michael R. Johnston):
> I'd say the consequences are very unpredictable unless you unmount the
> file system you are dumping. If the file system is active there is no 
> telling what will happen. It somewhat akin to trying to paint a
> picture of a moving target.

   I can be a bit more specific about what the "moving picture" contains
for the specific case of BSD4.2 dump/restore as shipped in SunOS 3.0
through 3.5:

  The program makes two initial passes collecting file and directory
information, then copies information to tape.
  There are really only wo places (well, times) when the dump
program can be confused about what to do. The first is between the
collection phase(s) and the copy phase, and the second is during
the copying of individual files (and therefore directories).
  If one adds a file between the scan phase and the copy, it will
not be backed up.  If one deletes one between those two phases, it
will not be backed up (and restore will complain about it).
  If, however, one deletes a file during the copying of that file,
it is liable to be copied, in part or in whole, to the tape.

  The "in part" is fun!

  In fact, certain editing programs (our, for example), manipulate
their current and previous-version files in such a way that dump
always gets one of the versions of an existing file being edited,
and files deleted tend (but only tend) not to be dragged back by
restore.

  In a word, dump/restore are **subtle**.

  I still try to do level zero dumps on a quiet system, though: its
easier than explaining why some deleted files reappear and others
don't.

--dave