On Thu, 29 Dec 2011, Marcus Sorensen wrote:

quoted

Poster already said they're not partitioned.

Correct. using partitioning allows you to adjust the alignment, so for
example if the MD superblock at the front moves the start of the
exported MD device out of alignment with the base disks, you could
compensate for it by starting your partition on the correct offset.

Unless he has used the XP jumper, it's impossible to misalign MD if 
running without partitions, afaik.

If he's writing full stripes he doesn't need to calculate parity by 
reading. I'm not sure how the MD layer determines this though, unless 
he's adding a sync or o_direct flag to his test he should be writing 
full stripes regardless of the blocksize he sets.

I've seen MD do 10% reads in this situation, I believe the handling of 
this is not optimal and sometimes there will be reads. Niel can probably 
tell a lot more about what might be going on.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se