Hello Backuppc-users,

I stuck while trying to identify the suitable rsync parameters to handle large 
image file backups with backuppc.

Following scenario: I use partimage to do LVM-snapshot based full images of my 
virtual (windows-) machines (KVM) blockdevices. I want to save theses images 
from the virtualization server to my backup machine running backuppc. The 
images are between 40 and 60 Gigs uncompressed each. The time-window for the 
backup needs to stay outside the working hours and is not large enough to 
transfer the images over the line every night. I red about rsync's capability 
to only transfer the changed parts in the file by a clever checksum-algorithm 
to minimize the network traffic. That's what I want.

I tested it by creating a initial backup of one image, created a new one with 
only a few megs of changed data and triggered a new backup process. But I 
noticed that the whole file was re-transfered. I waited till the end to get 
sure about that and decided that it was not the ultimate idea to check this 
with a compressed 18 GB image file but this was my real woking data image and I 
expected it to work like expected. Searching for reasons for the complete 
re-transmission I ended in a discussion-thread where they talked about rsync 
backups of compressed large files. The explanation made sense to me: The 
compression algorithm can cause a complete different archive file even if just 
some megs of data at the beginning of the file hast been altered, because of 
recursion and back-references.
So I decided to store my image uncompressed which is about 46 Gigs now. I found 
out that I need to add the "-C" parameter to rsync, since data compression is 
not enabled per default. Anyway: the whole file was re-created in the second 
backup run instead of just transfering the changed parts, again.

My backuppc-option "RsyncClientCmd" is set to "$sshPath -C -q -x -l root $host 
$rsyncPath $argList+" which is backup-pcs default disregarding the "-C".

Honestly, I don't understand the exact reason for this. There are some 
possibilities that may be guilty:

-> partimage does not create a linear backup image file, even if it is 
uncompressed
-> there is just another parameter for rsync I missed which enables 
differential file-changes-transfers
-> rsync exames the file but decides to not use differential updates for this 
one because of it's size or just because it's created-timestamp is not the same 
as the prior one

Please give me a hint if you've successfully made differential backups of large 
image files.

Thank you very much,

Andreas Piening
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