Re: [BackupPC-users] rsync incremental-only backups for eternity

[Rob O. <ro...@bi...>]

Thanks for this info.  It was very helpful.  I'm still trying to wrap my 
head around the scheduling and some of the terminology in the config.pl 
file.

-Rob

Holger Parplies wrote:
> Hi,
>
> Carl Wilhelm Soderstrom wrote on 09.07.2007 at 08:12:18 [Re: [BackupPC-users] rsync incremental-only backups for eternity]:
>   
>> On 07/09 08:22 , Rob Owens wrote:
>>     
>>> I see that the only difference between rsync "full" and rsync 
>>> "incremental" backups is that "full" uses the --ignore-times option.  
>>> Under what circumstances would this option be desirable?  Seems to me 
>>> that doing incremental backups forever would suffice, but maybe I'm 
>>> missing something.  What is the risk associated with only performing 
>>> incremental backups with rsync?
>>>       
>
> http://sourceforge.net/mailarchive/message.php?msg_name=20070501024958.GM25826%40mail.parplies.de
> http://sourceforge.net/mailarchive/message.php?msg_name=20070615011832.GL25826%40mail.parplies.de
>
>   
>> The way I tend to think of it (and I may be completely off base here) is
>> that 'fulls' are like what you get if you just run the 'rsync' tool from the
>> command line with no fancy options. 'Incrementals' are a faster way to do
>> things.
>>     
>
> I believe 'rsync's default mode of operation is actually incremental-type
> (i.e. optimize based on modification times). That makes sense for
> interactive operation, as you can decide what you need each time.
> '--ignore-times' is, in general, rarely needed, and it is an expensive
> operation, much like '-H' (which is also not default, even in '-a'). If you
> want to be absolutely sure you're making an exact copy, you need
> 'rsync -aH --ignore-times', but usually 'rsync -a' will do (and be much
> faster). Sometimes you might need to leave out '-o' and '-g' if you require
> copies with identical content but different ownership. As always, you need
> to know what you want before you can select the correct options to achieve
> it. If your interactive invocation does *not* do what you expect, you'll
> hopefully notice and correct it.
>
> For use as an automatic backup tool you want an invocation which produces an
> exact copy of your data under all circumstances - for every machine, whatever
> data your users happen to keep there, however they manipulate it. In addition,
> you likely won't notice anything going wrong until you need to restore the data
> and the restored data is not correct.
>
> As backups are commonly done frequently and with large data sets, it may be
> necessary to speed things up at an acceptable cost. Incremental backup
> strategies based on modification times are common enough. The drawbacks are
> basically known (if not always taken into account). With rsync, such a strategy
> *does* speed things up significantly, so it's natural to adopt a well known
> term and its well known implementation, even though rsync "full" backups in
> BackupPC already have almost all the benefits of conventional "incremental"
> backups (storage and bandwidth wise).
>
>   
>> Always keep in mind tho that BackupPC is *not* using the rsync tool on the
>> server side. It's using the File::RsyncP Perl module.
>>     
>
> Meaning not all options it respects are visible in the remote rsync
> invocation, and not all rsync options are supported by File::RsyncP.
> The '--delete' options are examples of things the remote instance does
> not need to worry about. BackupPC will take note of deleted files
> although these options are not explicitly visible.
>
>   
>> On 07/09 06:31 , Jean-Michel Beuken wrote:
>>     
>>> in theory, in the version 3.x of BackupPC, we can take incrementals of 
>>> different levels ( $Conf{IncrLevels} )...
>>> but, my little experience shows that we don't gain a lot of time with 
>>> high level (with rsync) worse some incr takes more
>>> times that the full :-(
>>>       
>
> Some incrementals need to transfer more data than a full backup would in the
> same situation :-). Note also the cost of constructing a backup view which
> increases with the level of the backup.
>
> Regards,
> Holger
>
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