borg extract from related repos

[ThomasWaldmann]

Just to keep a borg2 idea:

Assume we have multiple related repos that get backups of basically the same source data sets:

• REPO_FAST (e.g. it is local, same LAN or on a local disk even)
• REPO_SLOW (e.g. somewhere remote on an internet server, low throughput, high latency)

The backup archives in these repos could be:

• totally or partly identical (e.g. someone used borg transfer)
• all slightly different (someone did just independent backups to both repos)
• same, but differently pruned
• a mix of all of this

Assume we want to extract an archive from the slow repo (e.g. because it is somehow newer/better than the ones in the fast repo), we could implement code for this:

borg --repo REPO_SLOW extract --other-repo=REPO_FAST ARCHIVE_SLOW ...

borg would then fetch chunks from the related fast repo if they exist there, but the result would be the same as if it fetched everything from the slow repo.

[ThomasWaldmann]

see also: #6584

[ThomasWaldmann]

Likely this is doable more easily / cleanly after we dropped support for old repos.