Hi Steve,
It looks like they are stored using reverse deltas. Maybe you’ve already seen
this from the V4.0 documentation:
* Backups are stored as "reverse deltas" - the most recent backup is always
filled and older backups are reconstituted by merging all the deltas starting
with the nearest future filled backup and working backwards.
This is the opposite of V3 where incrementals are stored as "forward deltas" to
a prior backup (typically the last full backup or prior lower-level incremental
backup, or the last full in the case of rsync).
* Since the most recent backup is filled, viewing/restoring that backup
(which is the most common backup used) doesn't require merging any deltas from
other backups.
* The concepts of incr/full backups and unfilled/filled storage are
decoupled. The most recent backup is always filled. By default, for the
remaining backups, full backups are filled and incremental backups are
unfilled, but that is configurable.
Additionally these tips might help apply deltas to the files and reduce
transfer bandwidth:
MySQL dump has an option ‘--order-by-primary’ which sorts before/while dumping
the database. Useful if you’re trying to limit the amount to be rsync’ed.
You’ll need to evaluate the usefulness of this based on db design.
If you’re compressing your database look into the “--rsyncable” option
available in the package pigz.
From: Steve Richards <b...@boxersoft.com>
Sent: Tuesday, November 20, 2018 04:34
To: backuppc-users@lists.sourceforge.net
Subject: [BackupPC-users] Large files with small changes
I think some backup programs are able to store just the changes ("deltas") in a
file when making incrementals. Am I right in thinking that BackupPC doesn't do
this, and would instead store the whole of each changed file as separate
entries in the pool?
Reason for asking is that I want to implement a backup strategy for databases,
which is likely to involve multi-megabyte SQL files that differ only slightly
from day to day. I'm trying to decide how best to handle them.
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