Andreas Piening <andreas.piening AT gmail DOT com> wrote
on 12/22/2010 09:08:28 AM:
> One additional question: You say that the number of incremental backups
> will slow down the backup process, which makes sense to me because
> backuppc needs to iterate over all incremental backups. But which
settings
> are the "best" if I want a history that goes more than 7
days in the past?
> For instance if I want to keep the last 14 days in the backup, is
it
> enough to set "FullKeepCnt=2" and leave "FullPeriod=6.97"
and
> "IncrPeriod=0.97"?
> Does "FullKeepCnt=2" still use pooling then? What I mean
is: Does it need
> twice the space as with "FullKeepCnt=1", or are there still
hardlinks used
> to address the unchanged files?
Hard links are used no matter what. The only
difference between an incremental backup and a full backup is that an incremental
backup only checks files it can detect as changed, and a full checks every
one. It is possible for a file to be changed and not detected as
changed, so a full catches this.
It has *NOTHING* to do with how much space a backup
takes. Both will take *exactly* the same amount of space (ignoring
the possibility of files that might be missed by an incremental).
If you want to be able to restore a file for each of the last 14 days,
you want a FullKeepCnt that meets your needs (at least 2, but I would recommend
something like 2,2,2--look at the documentation!), and an IncKeepCount
of something like 12. That will give you 14 days of daily backup.
Again, read the documentation. It is extremely
well done. With BackupPC, it's easy to say something like, "I
want one backup a day for 14 days, one backup every week for 6 weeks, 6
backups every 2 weeks, and 12 backups every 4 weeks (close enough to a
month). That would give you nearly 17 months of backups, and depending
on your deltas (the amount of data that changes daily), it might take only
a few tens of percent more space than a *single* backup!
(For the record, the configuration for that is: FullPeriod=6.97
[*Very* little reason to change this ever], IncPeriod=0.97 [Same here,
too], FullKeepCnt=8,6,12 [8 weeekly backups, 6 2-weekly backups and 12
4-weekly backups] and IncKeepCnt=12 [12 daily incremental backups plus
two of the 8 weekly full backups during that time=2 weeks of daily backups].
These are *not* in Perl syntax, but you could enter them into the
GUI as-is.)
> The more I think about it the more I understand
how much "magic" must be
> going on in the backuppc backend!
Actually not that much. Much of the magic is
actually in rsync; the only real "magic" in BackupPC is
that it can detect when a "new" file is actually the same as
any other "existing" file and eliminate the duplicated space.
(Not to minimize the value of BackupPC: I love
it to death. But it is not a black box: it's actually very
understandable. Still, major props to Craig for putting everything
together in an easy-to-use package!)
Timothy J. Massey
Out of the Box Solutions, Inc.
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