add on hold snapshot feature

[kapitainsky]

First - great simple script. Exactly what I was looking for.

At the moment finding common snapshots to protect them during pruning depends on both source and target datasets being present which does not have to be always the case and creates risk that pruning will delete last sent snapshot.

It would be great to add "hold" feature:

https://www.thegeekdiary.com/understanding-holding-a-zfs-snapshot-feature/

This way snapshot would be safe.

After successful sent hold can be removed and new snapshot marked as hold.

[kapitainsky]

tested it by replicating to pool on external disk

when external destination disk/pool is not present and I run ./zfsbud.sh --remove-old on source it can delete last common snapshot

and subsequent attempt to sync results with:

WARNING: No common snapshot found between source and destination. Add the --initial|-i flag to clone the snapshots to the destination.

The "hold" would prevent it.

[kapitainsky]

ok I really need it:) so PR will follow soon

[gbytedev]

@kapitainsky I missed this issue; sorry for keeping you waiting. This is very interesting and I'm happy to test and merge this if you are still inclined to create a PR. Same with other sensible changes you have made to your fork. 👍

[kapitainsky]

Thx. I think PR is in waiting:) I am happy if you have any comments. I have been using my modified version and it works perfectly