Re: [rdiff-backup-users] Question about restoration procedure

[Andrew Ferguson]

On Sep 16, 2008, at 12:19 PM, Pedro DeRose wrote:
> So if restoration is based entirely on increments, how does rdiff- 
> backup know backup/a was created on 1970-01-01 when there's no  
> increment for that date? I've looked at the files in rdiff-backup- 
> data, and the only thing I can find that ties backup/a to 1970-01-01  
> is the files statistics. Does restoration use that to find out when  
> files are created? If not, what does it do?

Hi Pedro,

Sorry for taking so long to answer your question.

Rdiff-backup uses the mirror_metadata files as a history of the  
repository. The mirror_metadata file for 1970-01-01 contains an entry  
for backup/a, which is how rdiff-backup knows when files are created.  
The mirror_metadata file is the key to rdiff-backup's process: it  
stores all the the information about all files in the repository, and  
is how rdiff-backup is able to preserve FS-defined metadata (eg, HFS+  
resource forks, permissions, etc.) across platforms and users.

When asked to restore a time T, rdiff-backup first determines how the  
mirror_metadata (MM) file looked at that time. To do this, it first  
first checks if there is a 'snapshot' of the MM at time T. A snapshot  
is a complete dump of the mirror_metadata at that time (all non-zero  
snapshots and diffs are gzip'd to conserve space, although this is  
configurable). If there is not a snapshot at time T, then it finds the  
earliest snapshot in the range (T, now]. There is always a snapshot at  
the most recent backup. After finding the earliest snapshot, it  
applies the reverse-diff files which bring the MM file from that time  
back to time T. Rdiff-backup stores a new snapshot after every 10  
increments, both as a disaster prevention and to try and limit the  
amount of time needed to restore very old increments.

The file_statistics file is mostly for the curious user and can be  
read by other programs. At the moment, rdiff-backup does nothing with  
it except write it. This behavior can be disabled with the --no-file- 
statistics option.

You may also wish to read the dated, but still applicable, page 'rdiff- 
backup file format' -- http://www.nongnu.org/rdiff-backup/ 
format.html       It has a useful overview, but does not go into all  
details.


best,
Andrew