raid5 that used parity for reads only when degraded

[Alex Izvorski <aizvorski@gmail.com>]

Hello,

I have a question: I'd like to have a raid5 array which writes parity data but
does not check it during reads while the array is ok.  I would trust each disk
to detect errors itself and cause the array to be degraded if necessary, in
which case that disk would drop out and the parity data would start being used
just as in a normal raid5.  In other words until there is an I/O error that
causes a disk to drop out, such an array would behave almost like a raid0 with
N-1 disks as far as reads are concerned.  Ideally this behavior would be
something that one could turn on/off on the fly with a ioctl or via a echo "0" >
/sys/block/md0/check_parity_on_reads type of mechanism.  

How hard is this to do?   Is anyone interested in helping to do this?  I think
it would really help applications which have a lot more reads than writes. 
Where exactly does parity checking during reads happen?  I've looked over the
code briefly but the right part of it didn't appear obvious ;)

Regards,
--Alex