Hi,

Adam Goryachev wrote on 2008-09-04 08:55:51 +1000 [Re: [BackupPC-users] Backup 
through slow line?]:
> [...]
> BTW, 2 x rsync incrementals of the same level will transfer more data
> than one full + one incremental. So for example, doing 6 incrementals
> followed by a full backup can in fact transfer a lot more data than
> doing 7 full backups.

a while ago, rsync incrementals used to be based on the last backup of the
next lower level (i.e. full backup for level 1 incremental), while rsync full
backups were based on the last backup of any level (i.e. the previous backup).
Craig was considering changing this so that any rsync backup would always be
based on the previous backup. I don't know if this has happened yet (can't
find anything in the 3.1.0 changelog), or even if Craig is still considering
it.

I would argue that for a full backup, it is ok to base it on the previous
backup (there is no risk to the integrity, and it will probably transfer less
data), while for an incremental the possibility of missing changes might be
mitigated by referencing the previous backup of next lower level rather than
the previous backup (but there are no guarantees, so it's only a vague
possibility; then again, what other meaning do multilevel rsync incremental
backups have?) - meaning I probably wouldn't change anything.

Presuming nothing has changed (yet), it would in fact be optimal (regarding
bandwidth) to use alternating full and incremental backups. Nevertheless, it
depends on the individual situation if "a bit less bandwidth" is worth "much
more disk accesses".

Regards,
Holger

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