RAID 1 or Hammer

[nntp.dragonflybsd.org]

Hi,

I'm curious if RAID 1 (mirroring) really helps to protect data loss. Of 
course if a whole disk "dies", RAID 1 has the advantage that
I have an identical copy. But what happens if only a sector of one disk
contains bad data. How can the RAID controller decide which is the
correct sector? Or would the disk detect such a case and return an error?

Instead of mirroring the disk on the block level, wouldn't a Hammer 
mirror (to a second disk) provide the same degree of data protection?
Of course it wouldn't provide the automatic fail-over a RAID 1 setting
provides, and there would be no improvment for reading blocks, but on 
the other hand it would stress the disk less, e.g. by mirroring only 
every 5 minutes (don't know if that matters at all).

The background is, that I am planning to spend my old server (running 
FreeBSD) a new huge disk and replace it with DragonFly :). The main 
reason to use DragonFly for me is to ease backups, which right now is a 
real pain (using rsync and hard-linking). ZFS would of course also 
provide that, but I assume that Hammer runs better on a P4/i386 with not 
too much memory.

Regards,

  Michael