On 4/24/2014 3:52 PM, backuppc AT kosowsky DOT org wrote:
> Bowie Bailey wrote at about 14:38:26 -0400 on Thursday, April 24, 2014:
>   > Think of incremental backups this way...
>   >
>   > Pros:
>   > 1) They finish running quicker.
>   >
>   > Cons:
>   > 1) They can miss backing up files in certain circumstances
>   > 2) They cause the same data to be transferred multiple times.
>   >      - Each incremental will transfer all the same data as the previous
>   > one plus any new changes
>   >      - The next full backup will also need to transfer all of that data
>   > again.
>
> Not quite.
>
> Only data changed/added since the previous level incremental needs to be
> transferred.

True, but I was trying to keep things fairly simple.  Besides, I don't 
think I've seen an example of a situation where multiple levels of 
incrementals were needed in BackupPC.

> So technically, if you have infinite levels of incrementals, then
> each changed/new file is transferred only once so there are no extra
> transfers beyond what all fulls would require.
>
> In reality, you want to have a limited number of levels (cycling back
> to the full), since having more levels of incremental will cause
> reconstruction of the latest backup to take longer since all the
> previous levels back to the full need to be scanned -- in particular,
> all the attrib files need to be read to see what files may have been
> deleted... This reconstruction is needed not just when you restore but
> every time you back up so that changes can be identified. In
> particular, it is *not* sufficient to just look at the previous
> incremental level to determine what files have changed.

And if the incrementals were dependent on each other, you would have to 
keep *all* of those incrementals.  None of them could ever be deleted 
without losing information.  This would cause your pool size to 
continuously grow since deleted files would never be removed.

> The bottom line remains the same. If your bottleneck is bandwidth,
> then do all fulls.

Exactly.

-- 
Bowie

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