On Sat, May 10, 2003 at 04:55:27PM +0200, Per von Zweigbergk wrote:
> Hi.
> 
> I'm going to be deploying a backup system for a small school with a 
> single file server, as well as possibly backing up some PC's remotely. 
> I have not had any experience before in deploying one.
> 
 ...
> My question is, though, how do you handle filesystems that are active?
> 

Some OS's and filesystems have a facility generally known as "snapshots".
Using this technique, the snapshot does not change, and is readonly, while
the regular filesystem continues.  A snapshot does not duplicate things,
it uses the same data unless they change, then the regular file system
uses the new data while the snapshot uses the old.  When the snapshot
is deleted, then the space for the old, changed data is recovered.

However, this have your cake and eat it too scenario is not generally
available and amanda has no builtin mechanism to use it.  Some amanda
users do customize their installations to use snapshots.  I hope to with
my Solaris systems as that OS does provide the snapshot facility.

I think you will find that most people just accept that some individual
files may not be backed up properly.  I'm not sure about BSD's dump, but
my experience with gnutar, in "my environment" is that the entire dump
has never been corrupted because of a file changing in the middle of a
dump, just the individual file.  Also gnutar seems to recognize that a
file has changed during the backup and reports this.  I see the messages
in my regular amanda reports and they are few ... in my environment.
And almost always they seem to be some kind of temporary file I would
not care about anyway; like an intermediate file collecting incoming mail
that was probably removed 2 seconds later.

Basically I'm saying that it is more a theoretical problem than a real one.
Again, that is in my environment.  YMMV!!


Note that databases generally can not be backed up cleanly.  It may be
necessary to stop the database or make a separate dump of it to back it up.


-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
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