Most backup admins in the NT/2000 world perform "full" backups every night.
Why? Restore reliability. John Martinez's answer about *why* was right on.
He doesn't "want to go searching for the last full and merge with an
incremental." About 10 million other backup administrators are right there
with John.

When we created Retrospect, we looked at how other file-based backup
software worked (Backup Exec, ARCserve, etc.), and we were amazed to see
that nothing had really changed since the days of DOS 2.x.

These programs today still rely on the file's archive attribute to determine
what to back up in a "new and changed files" backup. Their incremental and
differential backups save time and media during the backup process, but they
make restoring difficult, and exact restores IMPOSSIBLE. If you did seven
incremental backups, then you have to perform seven incremental restores!
Good luck!

That's why most anyone who has had to try to rebuild a server from
incremental backups now uses a strategy of full backups every night.

Well, Retrospect is smarter than that.

Retrospect can restore any volume in a single pass over the backup media,
even though it only performs "new and changed file" backups. This is
possible because Retrospect performs a more intelligent method of file
selection, ensuring that one copy of each unique file from the source is
copied to *each* backup set.

Here's an ASCII diagram of the differences between how Retrospect and other
backup software work.


|                       |
|        SOURCE*        |
|                       |
:      BACKUP JOB*      :
:                       :
: (full or incremental) :
|      DESTINATION      |
|                       |<------ Files tracked here (what got copied)
|     (backup media)    |

* Note: Changing either the source or the backup job type will affect
        what files are backed up. Changing the media HAS NO EFFECT.


|                       |
|        SOURCE*        |<------ Files tracked here. (Snapshots)
|                       |                          :
-------------------------                          :
            |                                      :
            |                    ..................:
            V                    :
-------------------------        :
|      DESTINATION*     |        :
|                       |<------ and here (Backup Set Catalog)
|     (backup media)    |

* Note: Changing either the SOURCE or the BACKUP SET DESTINATION will
        affect what files are backed up. Retrospect chooses files by
        matching Snapshots against the in-use backup set's catalog.
        Any file found on the source volume that doesn't match exactly
        a file already in the backup set is copied.

Retrospect doesn't know how to do a "full" or an "incremental" backup.
Instead, it always backs up the files that aren't already present in the
backup set. When the operator selects a "Recycle" or "New Media" operation
in Retrospect, they're only changing the media handling parameters; reuse
the same media/backup set or start with a new set of media entirely.

When it's time to restore, Retrospect knows what files were on the source at
each backup (from saved Snapshots), so it simply copies all the files from
the backup set that are needed to make the source look exactly like it used
to at any previous point in time, and only with one restore operation.

Taking this one step further, this design allows Retrospect to keep
multiple, complete backup sets, but requiring only one for a restore. Bring
a set back from a week in offsite storage, and Retrospect will only copy the
files to that set that were created or modified in the last week.

Compared to other Windows backup software, Retrospect gives you the speed
and media savings of incremental backups with the reliability and precision
of full backups. It has all of the advantages of both, with none of the

I could talk about this for a long time--it's a complicated topic. Hopefully
this makes some sense. Let me know.

Eric Ullman
Dantz Development

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