On Wed, Sep 04, 2002 at 10:50:17PM +0200, Manuel Bouyer wrote:

 > But in this case wouldn't they already be adjacent at the upper level ?
 > What we say here is that MAXPHYS for a raid-0 raid is the sum of the MAXPHYSs
 > of the underlying devices.

Why would they already be adjacent at the upper level?  The RAID driver
simply chunks up the larger transfer into smaller ones at the correct
offsets and fires them off.  (For a simpler version, look at ccd.c)

 > Hum, I didn't consider the case when the stripe unit size is smaller
 > than the underlying MAXPHYS. Really raidframe's MAXPHYS is n*stripe unit size.

Right, RAIDframe (or ccd) is going to get at most a MAXPHYS-sized transfer
in the current scheme.  Maybe it gets several of those.  It fires them off,
and when it does so essentially guarantees that the request for each component
device is going to be < MAXPHYS (for simple RAID-0, the best you can hope for
is min(interleave_size, MAXPHYS/num_components), right?)

 > So sure, merging requests at the request queue would help there too.
 > 
 > -- 
 > Manuel Bouyer <bouyer@antioche.eu.org>
 > --

-- 
        -- Jason R. Thorpe <thorpej@wasabisystems.com>