How does bitrot protection actually work?

mdgm wrote:
I don't think that's possible. If the checksums at the filesystem level are all bad then one would expect the checksums at the md level to all be bad as well.

I think I understand...

Just so I am clear, the process would be that the bit rot protection routine first checks the primary data, then, if it finds an error, it goes out to the redundant data location and checks THAT data.

It will only write from the redundant location to the original location if it finds a correct checksum at the redundant location.

If it finds an incorrect checksum at both locations (it will find both locations incorrect, since the memory is defective) then no data is written and an error is generated.

This is the way the ZFS system works and inspires confidence. Do I have it correct?

mdgm wrote:
In any case bitrot protection is a great feature, but backups are still important. No important data should be stored on just the one device.

I am with you. Minimum of three (3) copies with at least one off site.

However, you can easily corrupt multiple copies if your primary source gets damaged by the file system and you do not know. As an example:

-On day 1, I have (3) 1TB drives A, B, & C with the same info.

-On day 2, copy A is damaged systematically by the file system without me knowing (but the drive is fine with no SMART errors).

-On day 3, drive B fails in the normal way, so I copy my data from drive A to drive B.