1. Generate a list of files on both sides and the sizes & dates, and compare that ignoring any that have changed/appeared since before the last backup cycle started. Unless your backups are truly massive in terms of number of files this is practical to automate and run at least as often as your backup cycle, and this catches many system errors or simple failures of the backups to run at all.
2. Occasionally checksum the whole damn lot in your latest snapshot and the originals. This can take a lot of time (and expense of you are using child storage with read access charges) so you want to do it less often but it catches bit rot and similar issues. Again you have to skip files that have been touched since the start of the last backup cycle.
3. If you keep a checksum (or list of files with checksum) of each snapshot, occasionally pick one and verify it from scratch. As with hashing the latest snapshot this can be quite resource intensive for massive backups but is fine for mine. You can also just compare meta-data (files, sizes, dates) to a stored list which will catch some types of filesystem corruption affecting your older snapshots.
One of these days I'll might get around to tidy+documenting+publishing my scripts that run all this…
That's close to what I do[1]. The size and date comparison is done by rsync, and I keep a text file with all expected file hashes, so if there's any disagreement between copies I know which one to trust.
These hashes are also ordered so that the top files haven't been checked the longest; part of the script is to take the top N files, checksum them, and move them to the bottom of the list. This guarantees every file is checksum once per N days.
I also donwload a random file in every run, to make sure the connection is not broken.
My use case is personal photos and videos, so I also make sure that my local files are never changed.
And finally, I highly recommend Hetzner Storage Boxes. Not only are they dirty cheap while still giving you ZFS and samba access, you can actually SSH into the box and run simple commands on the files locally, like sha25sum, without paying for network transfers.
A couple of things I do:
1. Generate a list of files on both sides and the sizes & dates, and compare that ignoring any that have changed/appeared since before the last backup cycle started. Unless your backups are truly massive in terms of number of files this is practical to automate and run at least as often as your backup cycle, and this catches many system errors or simple failures of the backups to run at all.
2. Occasionally checksum the whole damn lot in your latest snapshot and the originals. This can take a lot of time (and expense of you are using child storage with read access charges) so you want to do it less often but it catches bit rot and similar issues. Again you have to skip files that have been touched since the start of the last backup cycle.
3. If you keep a checksum (or list of files with checksum) of each snapshot, occasionally pick one and verify it from scratch. As with hashing the latest snapshot this can be quite resource intensive for massive backups but is fine for mine. You can also just compare meta-data (files, sizes, dates) to a stored list which will catch some types of filesystem corruption affecting your older snapshots.
One of these days I'll might get around to tidy+documenting+publishing my scripts that run all this…