RAID 1 or Hammer

Mon Jan 12 14:23:03 PST 2009

Konstantinos Pachnis wrote:
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?

When the controller will try to perform an I/O operation it will fail on 
the faulty disk (the disk with the bad sector). As a result the 
controller will be able to decide which is the correct sector.
You're assuming fail-stop errors.  If it is a sneaking bit error or 
something else, it won't notice.

cheers
  simon
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