On 08/20/2012 01:34 AM, Stan Hoeppner wrote:

I'm glad you jumped in David.  You made a critical statement of fact
below which clears some things up.  If you had stated it early on,
before Miquel stole the thread and moved it to LKML proper, it would
have short circuited a lot of this discussion.  Which is:

I'm sorry about that, that's because of the software that I use to 
follow most mailinglist. I didn't notice that the discussion was cc'ed 
to both lkml and l-r. I should fix that.

Thus my original statement was correct, or at least half correct[1], as
it pertained to md/RAID6.  Then Miquel switched the discussion to
md/RAID5 and stated I was all wet.  I wasn't, and neither was Dave
Chinner.  I was simply unaware of this md/RAID5 single block write RMW
shortcut

Well, all I tried to say is that a small write of, say, 4K, to a 
raid5/raid6 array does not need to re-write the whole stripe (i.e. 
chunksize * nr_disks) but just 4K * nr_disks, or the RMW variant of that.

Mike.