RAID 1 or Hammer

[Erik Wikström]

On 2009-01-12 21:37, Konstantinos Pachnis wrote:
> On Jan 12, 2009, at 9:13 PM, Gergo Szakal wrote:
> 
>> "nntp.dragonflybsd.org" <mneumann at ntecs.de> wrote in message 496b8121$0$881$415eb37d at crater_reader.dragonflybsd.org">news:496b8121$0$881$415eb37d at crater_reader.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?
>>>
> 
> 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.

Only if the sector is so damaged that you can't read it, before it gets
to that state it might very well be readable (and writeable) but with
corrupt data. This is the reason that ZFS keeps checksums of the data.

-- 
Erik Wikström