Not to belabor the point, but I think part of the confusion is that in
the history of backups, they were once called "differential incremental"
(only changes since last backup of any kind), and "cumulative
incremental" (changes since last full), and Netbackup still uses this
terminology.
Legato Networker does not have any documented "differential". The
"incremental" level is basically a level beyond level 9 - a perpetual
"only changes since last backup". Otherwise it uses the dump-style
levels through which you can get all the old semantics. They also add a
"consolidate" which doesn't perform a backup, but instead merges all
non-full backups with subsequent backups and produces a new saveset
which is in essense a full backup.
Not that I trust Wikipedia, but it agrees with my "classic" definitions:
http://en.wikipedia.org/wiki/Incremental_backup
I've not used the other products mentioned, but I did find a page on
Brightstor ARCserve that seems to indicate it uses my classic definitions:
http://www.open-mag.com/1226339824.shtml
Bacula I believe uses my definitions as well (although they don't
document it well, but the page for in on Backup Central agrees:
http://www.backupcentral.com/components/com_mambowiki/index.php/Bacula
)
Amanda I think uses simple dump levels.
And I checked BackupExec - I expected it to have the same definitions as
NetBackup, but this support document says otherwise:
http://seer.support.veritas.com/docs/230247.htm
Since an Incremental resets the Archive bit, a subsequent Incremental
would only pick up files where they Archive bit was turned back on -
i.e. anything change since the last full OR incremental. The
Differential does not reset this bit, so if you only do Fulls and
Differentials, then the Diffs would back up everything since the last
Full. Throw in an Incremental, and this would wreak havoc with your
Differentials. I think this could be a MAJOR source of confusion with
backups on Archive Bit OSes.
NTBackup does the same:
http://www.argentuma.com/backup/software/windows-backup.html
So, aside from Netbackup using different terminology, my "classic"
definitions seem to hold true.
-Brian
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