Support fast delta backups of same volume to multiple archives

[tasket]

Problem

If the user has multiple archives in a backup rotation schedule, the following obstacles can arise:

• Error message loading main archive.ini if the archives have the same UUID and are mounted at the same path.
• LVM requires --remap to switch to another archive.

Possible solutions

• Suggest user change the UUID of alternate archive(s) (esp. when source volumes are not LVM).
• Detect when user switches to older related archive, find common session points between them, and 'catch up' the delta map using manifests from the intervening sessions before running the normal delta process. This can be much faster than re-scanning/remapping. Automatic detection might be feasible for LVM since LVs each have their own UUIDs and could be recognized across different archives.
• Support multiple archive-snapshots for LVM (looks very messy).
• Suggest an external copy-update procedure instead of direct backups to the alternate archives.
• Implement internal copy-update feature.

Related

• Create and update duplicate archives #199