Hi,

Kelly Sauke wrote on 2011-07-01 09:21:28 -0500 [[BackupPC-users] Recompressing 
individual files in pool]:
> I have a need to modify certain files from backups that I have in 
> BackupPC.  My pool is compressed and I've found I can decompress single 
> files using BackupPC_zcat.  I can then modify those files as needed, 
> however I cannot figure out how to re-compress those modified files to 
> be put back into the pool.  Is there a tool available that can do that?  

no. It's not a common requirement to be able to modify files in backups.
Normally, a backup is intended to reflect the state a file system was in at
the time the backup was taken, not the state the file system *should have*
been in or the state *I'd like it* to have been in. I sure hope you have
legitimate reasons for doing this.

If you are modifying files, you'll need to think about several things.

* Do you want to modify every occurrence of a specific content (i.e. all
  files in all backups linked to one pool file) or only specific files,
  while other files continue to contain the unmodified content?

* If you are modifying every occurrence of a specific content, you'll either
  have to find out which files link to the pool file (hard, with a reasonably
  sized pool) or ensure you're updating the content without changing the inode
  (i.e. open the file for write, not delete and re-create it). If you do that,
  there is not much you can do for failure recovery. Your update had better
  succeed.

* Does your update change the partial file md5sum? If so, you'll need to move
  the pool file to its new name and location. Presuming the new content
  already exists, you should probably create a hash collision. That may be
  less efficient than linking to the target pool file, but it should be legal
  (when the maximum link count is exceeded, a second pool file with identical
  content is created; later on the link count on the first file may drop due
  to expiring backups), and it's certainly simpler than finding all the files
  linked to your modified pool file and re-linking them to the pre-existing
  pool file.

* If you're only changing individual files in a pc/ directory, the matter is
  far more simple. You'll need to take some code from the BackupPC sources
  for compressing anyway, so you might as well take the part that handles
  pooling as well (see BackupPC::PoolWrite and note that you'll be coding in
  Perl ;-).

> Is there a better way to go about modifying certain files within my 
> backups?

Once the contents you want to have in the files are in the pool (probably from
a recent backup), you can just figure out the pool files and link to them. If
you want that to be really easy, ask Jeffrey about his hpool directory ;-).

Regards,
Holger

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