Hi,
Matthias Meyer wrote on 2011-04-03 20:06:31 +0200 [Re: [BackupPC-users] Keeping
1 month of files and number of full backups]:
> Scott wrote:
>
> > I want to be able to restore a file for users up to one month in the past.
> >
> > What is the difference / what is the best -
> >
> > To do a full backup every 2 weeks, keeping 2 full backups,
> > and incrementals every day , or
> >
> > Do a full backup every 1 month and incrementals every day?
>
> Within the Web GUI of BackupPC are no differences bwetween incremental or
> full backups.
correct. Incremental backups appear "filled". You see every file with the
content it had at the time of the backup, regardless of where this content is
stored within BackupPC.
Concerning space requirements, the differences between full and incremental
backups are negligible.
> [...]
> If you are using rsync than the only difference between full and incremental
> are:
> - incremental only scan files which are created after the last backup.
Not true. *rsync* full *and* incremental backups will both transfer any files
they determine to have changed or been added, using a full file list for
comparison of the states on client and server. They also both "remove" deleted
files from the backup. The difference is that "incremental" backups only check
file attributes (particularly the timestamps) for determining changes (which
is sufficient except for extremely rare cases), whereas "full" backups check
file contents, i.e. they really read every single file on the client (and the
server, usually). This takes time and probably puts wear on the disks.
So, true:
> Therefore it is a lot faster than a full backup.
> - full backup scan all files, also files which would be extracted from an
> archive after the last backup but have an extracted timestamp older than
> the last backup.
> Therefore a full is much slower but get really all "new" files.
See above. The difference described is true for *non-rsync* backups. tar or
smb backups only have one timestamp as reference - that of the previous
backup, not the timestamp of every individual file - so incrementals can only
catch modifications (or creations) with timestamps later than the previous
backup. File deletions are not detectable by non-rsync incrementals, meaning
deleted files will continue to show up in your backups in the state they were
last in until you run a full backup.
Also note that full backups will, due to the full file comparison, correct
the probably extremely rare event of pool file corruption by making a new copy
with the correct content (past backups will, of course, not be corrected).
Someone please feel free to add a note on the effect of checksum caching
(turned off by default) ;-).
There is no one general answer to your question, "what is best". It depends on
your requirements. For probably all backup solutions, full backups are more
exact than incremental backups. Incrementals are a compromise for the sake of
practically being able to do backups at all (imagine the amount of tapes you
would require for full tape backups each day - and what small amount of that
data really changes).
BackupPC is designed for saving space used by redundant data, so you *could*
do daily full backups almost without penalty. Saving time is the motivation
here for copying the semantics of incremental backups. With *rsync*, the
difference in backup exactness is small enough to be a theoretical matter
only. For tar/smb, that is not true.
So it's really your decision. How much exactness do you need, how much can you
afford? For what you describe, you're unlikely to make a wrong decision.
Anything from daily full backups to one monthly full and daily incrementals is
likely to work for you. Just don't be surprised about the extra full backup
(any full backup an incremental backup depends on must be kept, so you will
almost always have 1 more full backup than you requested).
One thing to keep in mind, though: if network links with low bandwidth are
involved, you will want to do rsync-type backups (rsync or rsyncd), and
frequent full backups will actually use *less* bandwidth than long FullPeriods
and frequent incremental backups.
Regards,
Holger
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