RAID-3?

[Scott Long]

Greg 'groggy' Lehey wrote:
> On Wednesday, 18 August 2004 at 23:44:01 -0700, John-Mark Gurney wrote:
> 
>>I originaly was working on a RAID-3 module (which is possibly where
>>pjd got his idea) that used Luigi's FEC code.  The advantage of this
>>code was the fact that you could have n parity disks beyond the m
>>data disks.  The advantage of this was that you could loose any n
>>disks, and your data is still recoverable.  Unlike with RAID-4/5
>>implementations where if you happen to loose a second disk (due to a
>>power surge or something) while rebuilding, you'd be SOL.  That type
>>of redundancy is good thing to have.
> 
> 
> I can see that as a great advantage, but it's not part of the RAID-3
> definition, and I can't see why you couldn't expand RAID-5 in a
> similar manner.  Am I missing something?
> 
> Greg

Yes, you are!  The advantage of RAID-3 is that there are NO
Read-Modify-Write cycles when writing blocks.  Period.  Zippo.  None.
Every write takes exactly the same amount of time.  There is no waiting
for data to be read off of any disks.  That is why it's nice to
applications that require fixed latency.  RAID-3 has no concept of
stripe sizes becuase of this, unlike 4 and 5.

Scott