On Sun, 07 Jul 2013 19:39:27 +0200 Milan Broz [off-list ref] wrote:

So if you have multiple IOs submitted in parallel from *different* CPUs,
they are processed in parallel.

If you have MD over dmcrypt, this can cause problem that MD sumbits all IOs
with the same cpu context and dmcrypt cannot run it in parallel.

For RAID1 and RAID10 this isn't true any more.

Commit f54a9d0e59c4bea3db733921ca9147612a6f292c
in 3.6 changed this for RAID1 and a similar commit did for RAID10.
RAID4/5/6 still submit from a single thread as you say.

NeilBrown