Read-Only RAID-5 on DVD? :-)

[James Gray]

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Daniel Carrera wrote:
> On Mon, 2006-03-07 at 10:56 +1000, James Gray wrote:
>> If it's not sequential, it
>> wont work. RAID5 is non-sequential.  Game over.
> 
> Maybe you can expand on how RAID-5 is not sequential. When I read the
> OP's post my first reaction was that it couldn't possibly work, but here
> is a convoluted thought:
> 
> * Start making 3 raw images: (A, B, C)
>   - Put a block in A, a block in B and the parity in C
>   - Put a block in A, a block in C and the parity in B
>   ...
> 
> You get the idea. Essentially a "RAID 5", except that you are writing
> the blocks to some on-disk file - would probably have to be some other
> disk, not the one you are backing up...
> 
> * Then make an ISO of each of those images and burn them.

Except that RAID5 rarely *actually* writes blocks out in sequence as you
describe above - due to write caching etc.  As long as one block and the
parity is written, you can omit writing the second block until that
drive is idle (or a max cache flush interval is reached).  This is how
RAID5 is implemented in the Linux kernel and early every other RAID5
implementation I've seen (both software and hardware).  So consequently
the only way to write out as you describe is either to hack some funky
RAID5-for-ISO9660 or hack a new ISO9660 that writes across multiple
volumes as you describe above.

Like I said, it would be more of a hacked ISO9660 than a RAID5
implementation.

Hope that makes sense....it's been a long day :P

Cheers,

James
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