Hi Flavio,
On Thu, Jun 04, 2009 at 08:23:36AM +0200, Boniforti Flavio wrote:
> > Q: Does anyone know?
> > A: Yes
>
> Maybe I'm the only human which thinks a bit more "elastically", but if
> anybody asks:
>
> "Anybody knows why there's difference between the above 2 values?"
>
> I'm prone to explain *why* there's a difference and not wasting my time
> and the time of others for typing simply "yes".
> But that may be a weird and complex way-of-thinking that is affecting
> only me :-/
Nope, I tend to think like that as well. :-|
> > Asked and answered. What you were really looking for was:
> >
> > Q: Please explain to me the difference between the two number?
> > A: The file structure under backuppc has little resemblance
> > to the file structure of the original system. For each
> > directory, there is essentially a file with the directory
> > information as it appears on the original system (to handle
> > permissions and such). This will cause the numbers to differ
> > across multiple directories. There may be more reasons, but
> > the question seems so arbitrary and pointless I dont care to
> > put a lot of effort into getting a definitive answer. Maybe
> > if you have a good reason why you care why the numbers are
> > different I might be more interested.
>
> The reason which *for me* is worth knowing to which value I have to
> trust and *why*, is the fact that I have to account for HDD space usage.
> I'd really be happy using the values that BackupPC shows in the "Host
> Summary", but if they ain't really what HDD space usage should be, I
> just want to know which value to consider. If anybody has already done
> this sort of considerations (HDD space accounting per single host, which
> corresponds to a single customer), please explain to me his or her
> considerations.
The short answer is: You cannot account for single host disk usage
because of pooling.
Suppose, you've got three hosts, all with the same operating system. All
common files will be in the pool only once. And they will get hardlinked
every time you do a full backup.
So you get three hosts, with, say, 5 backups each. The /boot/vmlinuz
file will be shared by all those 15 backups. How would you account disk
space?
And it get's more complicated in practice. Customer 1 installes Firefox,
Customer 2 and 3 use Opera. So, now only 2 and 3 share the common
Opera files in the pool (which might be compressed after all).
Even if you developed some formula how to account for disk space, it
is very expensive to figure out who shares a common pool file - you'd
need to scan all pc/ directories and remember inode numbers etc.
My advice: Just account on total amount of data backed up. And this is
the number you get in the host summary page as "Full Size(GB)" and in
host status page in the "File Size/Count Reuse Summary" table in
"Totals" column - here you can also see the pooling effects nicely.
HTH,
Tino.
--
"What we nourish flourishes." - "Was wir nähren erblüht."
www.lichtkreis-chemnitz.de
www.craniosacralzentrum.de
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