Hi,

Pavel Hofman wrote on 2011-05-31 15:24:56 +0200 [[BackupPC-users] Rsynv vs. 
tar, full vs. incremental]:
> Incremental backup of a linux machine using tar (i.e. only files newer
> than...) is several times faster than using rsync.

that could be because it is missing files that rsync catches. Or perhaps I
should rather say: yes, tar is probably more efficient, but it is less exact
than rsync, because it only has one single timestamp to go by, whereas rsync
has a full file list with attributes for all files. One very real consequence
is that tar *cannot* detect deleted files in incremental backups while rsync
will.

My understanding is that the concept of incremental backups, way back in times
where we did backups to tapes, was introduced simply to make daily backups
feasible at all. Something along the lines of "it's not great, but it's the
best we can do, and it's good enough to be worthwhile".

Nowadays, "incremental" backups still have their benefits, but we really need
to shake the habit of making compromises for no better reason than that we
haven't yet realized that there is an alternative.

If you determine that incremental tar backups are good enough for you (e.g.
because the cases it doesn't catch don't happen in your backup set), or that
your server load forces you to make a compromise, then that's fine. But if
it's only "tar is faster than rsync and faster is better", then you should
ask yourself why you are doing backups at all ("no backups" is an even faster
option).

> On the other hand, full backup using tar transfers huge amount of data over
> network, way more than the efficient rsync.

There are also other factors to consider like CPU usage. Where exactly is your
bottleneck?

> Is there a way to use rsync for full backup and tar for the incremental
> runs?

No. Actually, *the other way around*, it would make sense: full backups with
tar (probably faster than rsync over a fast local network - depending on your
backup set) and incremental backups with rsync (almost as exact as a full
backup).

> I do not even know whether the two transfer modes formats produce
> mutually compatible data in the pool.

No. There is (or was?) a slight difference in the attribute files, leading to
retransmission of all files on the first rsync run after a tar run (because
RsyncP "thinks" the file type has changed from <something> to plain file).
The rest is, of course, compatible. It would be a shame if pooling wouldn't
work between tar and rsync backups, wouldn't it? :)

Regards,
Holger

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