You are welcome!
About the stress test: I hope, you'll document your method in detail. "Numbers" are very useful in the disucssions here rather than just saying "slow" or "fast".
I/O of the database as the bottleneck: This is an important finding. People often replace parts with "faster" components, without knowing whether they are critical or not. In your case, the result does not surprise anybody: see the post http://moodle.org/mod/forum/discuss.php?d=211249#p921345
(it was in the discussion I've linked in my earlier post).
About your strategy:
> 1) use SSD SAN storage
Theoretically yes. Practically I don't know.
> 2) MySQL cluster
People have come up with various architectures, but this forum lacks concrete numbers and a clear documentation on which implementation. Others suggest goint to PostgreSQL
, still no further information. You can use the 'Advanced serach' in the intro to find them.
> 3) setting up multiple read-only MySQL server (next to the single writable MySQL server)
Never heard. Still the writable database will dictate the speed, won't it?
4) Physical database server: See http://moodle.org/mod/forum/discuss.php?d=211249#p921345
(the same link as above).
5) Debug the database queries and give feed back to the developers. See P.S. in http://moodle.org/mod/forum/discuss.php?d=211679#p922637
. Optimising the application itself has the biggest impact.
6) Revert to 2.2, or to 1.9 or even to 1.6 if you use quizzes in a massive scale.
You are going to need some drastic measures since you are still far away from the target.