Hi,

Daniel Carrera wrote on 2009-05-23 08:38:01 +0200 [Re: [BackupPC-users] "Full" 
backup]:
> Holger Parplies wrote:
> > 1.) That is what you are requesting BackupPC to do.
> >     [...] An incremental backup *can* miss changes. That is highly
> >     unlikely with rsync but remains possible. [...]
> 
> How can it miss changes?

rsync compares file attributes (most importantly file size and modification
time). If they exactly match, it assumes the contents are unchanged. That is
almost always true, but it *is* possible for this to be wrong.

> How do full backups fix it?

They --ignore-times, thus comparing the contents even for apparently unchanged
files. But you know that. It was the initial question of this thread.

> > 2.) Backup dependencies
> >     A level 1 backup cannot depend on any other level 1 backup (because this
> >     other backup can - and probably will - expire first).
> 
> This is something else I don't understand. Why does it create 
> dependencies?

Les already answered this, but let me rephrase it: it is an optimization
internal to BackupPC that
- speeds up the critical and common operation of taking backups,
- conserves file links, delaying the need to duplicate content due to
  exhausting the possible number of hardlinks to one inode,
- conserves inodes due to not backing up unchanged 0 byte files (which aren't
  pooled) in incremental backups, and
- conserves disk space by making the directories smaller than they would be
  if they contained links to all files and not just changed ones.

"Internal to BackupPC" means that you don't have to worry about it. BackupPC
knows how/when to expire full and incremental backups to keep dependencies
satisfied.

> I thought that BackupPC did something that was functionally 
> equivalent with its pooling thing.

It does. The implementation is simply a bit different.

Regards,
Holger

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