Re: [PATCH 09/18] md/raid6: remove expectation that Q device is immediately after P device.

[Bill Davidsen <davidsen@tmr.com>]

NeilBrown wrote:
> Code currently assumes that the devices in a raid6 stripe are
>   0 1 ... N-1 P Q
> in some rotated order.  We will shortly add new layouts in which
> this strict pattern is broken.
> So remove this expectation.  We still assume that the data disks
> are roughly in-order.  However P and Q can be inserted anywhere within
> that order.
>   

Is this a change which could be done on the fly? Because if it is, 
there's obviously a huge performance gain to be had in degraded mode.

To clarify, if the order could be changed (raid5 example):
    running array         0 1 2 3 4 P
    failed device          0 1 F 3 4 P
Leads to a recover of data from the failed device. But if the order 
could be changed on the fly, then "recover" would be a one time 
operation, recover the entire chunk previously on device 2, write to 
device 5(P), change the order so the parity is on the failed device 
(can't recover from another fail anyway), and:
    remapped stripe    0 1 F 3 4 2

Now reads will not cost a recover, and writes hopefully could skip the 
parity completely. When the failed device is replaced the rebuild could 
restore the chunk order to improve leveling of head motion.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark