Hi,

Christian Neumann wrote on 2009-09-28 15:57:24 +0100 [[BackupPC-users] 
"Exponential expiring incremental backups" with IncrKeepCnt?]:
> [...]
> The documentation mentions exponential expiring incremental backups
> (http://backuppc.sourceforge.net/faq/BackupPC.html#backup_basics): "BackupPC
> can also be configured to keep a certain number of incremental backups, and
> to keep a smaller number of very old incremental backups."

while I don't really understand what "keep a smaller number of very old
incremental backups" is supposed to mean, there is no mention of exponential
incremental backup expiry. If you read the preceeding paragraph on full
backups you'll notice that it's described very explicitly there. If there were
exponential expiry of incrementals, there would be at least a clear reference
to this description.

> [...]
> Are "exponential expiring incremental backups" supported? If not, is there a
> reason behind it?

Exponential expiry of incremental backups really makes no sense (and it's not
sanely implementable with multi-level incrementals anyway). With BackupPC, you
*need* regular full backups(*) (if the wiki were functional, there would
probably be a page explaining why), and storing full backups is only
insignificantly more expensive than storing incrementals anyway. For this
reason, incremental backups are always fairly young (mine are up to 60 days
old, and I doubt anyone keeps them much longer). To keep an incremental backup,
you also need to keep the full backup it was made against(!), so the age
difference between the incremental and its full will never exceed
$Conf{FullPeriod} (the time between two full backups). With exponential
incremental backup expiry, you would quickly exceed $Conf{FullPeriod}, meaning
you would be keeping full backups (if only to support the incrementals) that
are closer to the incrementals than they are to each other. Why would you want
that?

Incremental backups are there for gaining a speed advantage - an advantage
that will allow you to make daily (or hourly or whatever) backups. Full
backups are (amongst other purposes) for keeping exponentially - yearly
backups for the last 10 years, monthly for the last two years, weekly for the
last six months (just to give you an idea). As with any backup system,
incremental backups are only a (good enough) approximation. Only full backups
give you a true snapshot (and that only if they are, in fact, taken of a
snapshot, but that's a different topic). You want to keep true snapshots
around for a long time, not approximations.

Regards,
Holger

(*) Actually, you probably need regular full backups with any backup scheme.
    It's just that on this list, we make a point of telling you ;-).

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