[BackupPC-users] Another thought on incremental backups of backuppc

I mentioned yesterday that the approach used in BackupPC_copyPcPool to
create an in-memory inode hash of the pool which I currently use to
copy an entire BackupPC implementation can be extended to copy
individual backups and thus in a sense perform incremental backups
across two different BackupPC trees with different pool contents (and
inode numbering).

This led me to think that one of the big current challenges with any
notion of incremental backups is that pool chain renumbering creates a
problem since there is no guarantee that a chain file has the same
suffix after BackupPC_nightly runs. While I can get around this by
creating a mapping between old and new pool file inode numbering (and
naming), I realized that a small tweak to BackupPC_nightly would make
such notions easier.

If BackupPC_nightly instead of deleting files with one link, just
zeroed them, then the space would be recaptured without any need for
renumbering. In theory this would have only two small negative
effects. First, you recover the space but you lose an inode. Second,
chains will over time get longer slowing the resolution of md5sum pool
name ambiguities when the whole chain needs to be walked (though zero
size files should take minimal time to walk -- just a 'stat'
really). Since chains are relatively rare, the cost may be worth
bearing even though the solution is inelegant. But at least it would
guarantee that the name and inode of a non-zero pool link would not
change.

Now I am not interested in going that direction because of it's
inelegance, but it would solve some problems and open up some new pool
syncing and backup solutions...

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