RE: [BackupPC-users] Backups over slow connections

[Hamish G. <ha...@pr...>]

Josh,

Thanks for your input relating to this. I am hoping to 'go live' with a
BackupPC backup of one of my new remote servers tomorrow. I accept that it
will probably take a while to make the first backup - it is a 256k link and
the server has about 6GB of data! In fact, the first backup of remote sites
is an issue, but this is another topic, slightly related, but more about
that later. I have just completed the rsync mirror of this site, and need to
set up the BackupPC files, so hopefully this will not take too long.

I have not yet looked at the algorithm within BackupPC which does the rsync
version of a backup, but empirically, I am assuming there is something going
awry. Maybe it is only on my local site, but that is still to be determined.

I will certainly post results once I have the new system is up and running.

Regards

Hamish




-----Original Message-----
From: bac...@li...
[mailto:bac...@li...]On Behalf Of Josh
Marshall
Sent: 11 February 2005 12:30
To: Backuppc-Users
Subject: Re: [BackupPC-users] Backups over slow connections


Hamish Guthrie wrote:

>Hi Josh,
>
>I do not understand why then some of my local PC's take so long to make
>backups using BackupPC, I have not yet checked to see what is actually
>transferred across the network, but, for instance, I have a local PC with
>47G of data which frequently takes 437 minutes to do a full backup, and
>incrementals take in excess of an hour on a 100Mbit/s LAN, yet for the same
>PC, using raw rsync, over the same LAN the time expended is a mere few
>minutes (like 2 minutes!).
>
>
A few things to point out:

1. Rsync is a binary. BackupPC is running Perl.
2. Rsync is comparing two physical sets of files. BackupPC has to deal
with pooling etc.
3. Rsync has the advantage of being able to detect whether a file has
changed by using the file information such as last modified time. Since
BackupPC doesn't store the files with this information in the pool (as
the pool contains files of identical content and not identical modified
times etc) BackupPC must check that the files checksums match up.

I believe that you'll find if you do tests that the bandwidth of
BackupPC using rsync will be slightly higher than using "raw rsync".
That said it will be much lower than if you were using tar which can't
send parts of a file.

You will also find that during the backup, the BackupPC server is
hitting the hard disk fairly hard. The delay isn't the data being sent
rather all the processing the server needs to do to back up the machine.
After your first backup, you'll find that backing up over a slow
connection will take about the same amount of time as on a 100mbit LAN
if there are minimal changes.

Please post your results of bandwidth up on this list so we can see a
good comparison of "raw rsync" vs "BackupPC rsync"

Regards,
Josh.


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