[techtalk] RE:backup newbie

[Eric Richard Turner]

On Fri, 1 Dec 2000, Brian Sweeney wrote:
> 
> >Good question. I suspect it depends on the backup utility and/or operating
> >system. I know that NTBACKUP on Windows NT uses the archive bit on each
> >file. During a full-system backup it clears the archive bit on all files.
> >When a file is changed, the archive bit is set. Differential and
> >incremental backups save only those files for which the archive bit is
> >set. The difference between the two is that during differential backups,
> >the archive bit is not cleared whereas during incremental backups the
> >archive bit is cleared.
> >So, using NTBACKUP on Windows NT, the answer to your question is no.
> >However, it seems reasonable that one could write a backup utility that
> >would catalogue when each file has been saved, perhaps using a checksum,
> >CRC or last-modified date to determine when a file has been modified. Then
> >the backup utility could do all sorts of interesting backup scenarios
> >based on the combination of the catalogue and the list of modified files.
> >Whether or not this sort of thing exists is beyond the scope of my
> >experience. I've never had to set up incremental or differential backups
> >for Linux, so I'm not sure what can and cannot be done. I recently
> >received the latest Linux Journal, which has an article in it about
> >automated backups in Linux. Now I'm curious, so I guess I'll just have to
> >read it :-)>>>
> 
> I'm not sure that's completely correct; with Linux's dump, for instance, you
> specify the level of the backup (0-9) when creating an incremental backup,
> so it's not neccesarily what's changed since the day before.

Since I've never had to implement backups under Linux, my example was NT.
Linux is a completely different animal! With NT and the NTBACKUP command,
what I wrote is correct. The NT BACKUP utility uses the archive bit on
files to determine which files need to be backed up. Each time a file is
modified, the archive bit is set so that the backup utility will back up
the file. During the next backup, if the backup is an incremental then the
archive bit is cleared so that the file doesn't automatically get backed
up each during subsequent backup. If the backup is differential then the
archive bit is NOT cleared so that the file keeps getting backed up
everytime until the next full-system backup (at which point all files'
archive bits are cleared).

>With dump, as
> with many archive utilities, , the difference between an incremental and
> differential backup is that a differential backs up everything changed since
> the last FULL BACKUP, whereas an incremental backs up everything changed
> since the most recent lower-number backup. So, a differential would look
> like this:

Yes, this agrees with my definition of incremental and differential,
except that NT doesn't have options for various levels: it's all or none!
Yet another case for the superiority of Linux as a server OS *grin*.

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