Very inconsistent (read) speed on UFS2

[Lev Serebryakov]

Hello, Kirk.
You wrote 31 августа 2011 г., 3:00:45:

> So, my new theory on why your O_DIRECT test is running slowly is due
> to the single threading in the GEOM layer. Pawel Jakub Dawidek (pjd@)
> gave a very interesting talk on this problem at this year's BSDCan.
  I want to stress my point: it is not low speed, what especially
 bother me. It is inconsistency.

  Bad drive, always-single-threaded-GEOM, etc., should give consistent
 slow speed.

  Bad code in geom_raid5 could give inconsistent write speed, du to
caching, but reading path is as straight and simple as possible here.

  Bad drive or AST-GEOM is not what simple to fix (ok, bad drive IS
simple to replace, of course). But I suspect, that here is some simple
"misunderstanding" between geom_raid5 code and VFS/FFS2 layer, which I
could fix. But I don't know where to look at.

-- 
// Black Lion AKA Lev Serebryakov <lev at FreeBSD.org>