Re: [BackupPC-users] Newbie: full vs incremental backup

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Msquared wrote:
> On Sun, Sep 28, 2008 at 08:02:56AM -0400, Andrei Stebakov wrote:
> 
>> Now it looks like every incremental backup is not different from a full
>> backup as it takes almost the same space on disk as a full backup. I
>> thought it should only backup the delta...
> 
> I think that while a delta is backed up, BackupPC then uses hard links to
> the files from the last backup in order to fill out the backup so that it
> looks like a full copy.
> 
> I also think this is part of BackupPC's pooling mechanism.

When a backup is in progress, (an incremental using rsync) rsync reports
the file has changed, so it copies the old file into
backuppc/pc/<hostname>/<num>/<directory>/<filename>
It then updates this file with the changes.
Once the complete backup has finished for this host, then the
backuppc_link process runs through the files and hard links
backuppc/pc/<hostname>/<num>/<directory>/<filename> to backuppc/pool/<hash>

Thus, if you do a du -s backuppc/pc/<hostname>/<num>/ you will get the
full size of the backup, however (on linux, in my experience) if you do
a du -sm backuppc/pc/<hostname>/1?? it will show something like this:
323545  backuppc/pc/<hostname>/100
112     backuppc/pc/<hostname>/101
105     backuppc/pc/<hostname>/102

This shows that the first backup used 323545M of space, while the next
one only used 112M because most of the files are sharing the same space
on disk, hence du doesn't double count them.

> For comparison, I theoretically have 1terabyte of data backed up, but it's
> actually only consuming about 120-130gigabytes of storage on my backup
> server, thanks to BackupPC's data pooling.

Yep, exactly, the files are only stored in the pool but are hard linked
into each of the host/backup directories where they are needed.

Regards,
Adam
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