In short, fsck simply checks to see that the metadata makes sense, and that all inodes belong to files, and that all files belong to directories, and if it finds any that don't, it attaches them to a file with a number for a name in lost+found.
It's pretty crude compared to a filesystem debugger.
If you want to compare apples-to-apples, you'd be better off asking how zdb compares to debugfs (for ext2/3/4) as both are filesystem debuggers.
You could also ask "How's zfs scrub different from fsck?" and the answer to that would be: zfs scrub checks every bit of data and metadata against saved checksums to ensure integrity of everything on-disk. In comparison, fsck cannot detect data corruption at all, and can only detect metadata corruption when an inode points to an illegal disk region (for example).
Even that comparison shows fsck is crude when compared to scrub.
The tool to recover from corruption is a rollback:
usage:
clear [-nF] <pool> [device]
That's answered very well in the article connected to this other currently active HN discussion:
https://news.ycombinator.com/item?id=5460988
In short, fsck simply checks to see that the metadata makes sense, and that all inodes belong to files, and that all files belong to directories, and if it finds any that don't, it attaches them to a file with a number for a name in lost+found.
It's pretty crude compared to a filesystem debugger.
If you want to compare apples-to-apples, you'd be better off asking how zdb compares to debugfs (for ext2/3/4) as both are filesystem debuggers.
You could also ask "How's zfs scrub different from fsck?" and the answer to that would be: zfs scrub checks every bit of data and metadata against saved checksums to ensure integrity of everything on-disk. In comparison, fsck cannot detect data corruption at all, and can only detect metadata corruption when an inode points to an illegal disk region (for example).
Even that comparison shows fsck is crude when compared to scrub.
The tool to recover from corruption is a rollback: usage: clear [-nF] <pool> [device]