Large HDD cache: how to fine-tune Fsys?

I look into the disk throughput on my laptop.
There are two HDDs in the test: Toshiba MK2023GAS and Hitachi
QNX v4.25G with Fsys.atapi, no other programms running.

I use “cp -V /dev/hd0t77 /dev/null” as a gauge.
No matter the Hitachi’s HDD has 7200rpm and 8MB cache - both HDDs show the
same ~17MB/sec “speed”. Mr.Krten’s “sysmon” reports that CPU load is ~40%
in Fsys and ~4% in Fsys.atapi for both HDDs.

How to read those results?
Definately, both are on UDMA mode now, for example in PIO mode the system
load is exactly reversed between the modules: ~40% Fsys.atapi and ~4% Fsys.

If I look into this with winXP - I see HDDs are UDMA2 both, but I do not
know of a way to get figures (MB/sec). The overall feeling is winXP is
much faster with the Hitachi’s HDD.

How to get the similar improvement for QNX4?

How to fine-tune Fsys’s cache size|policy in the presence of the HDD’s 8MB
Isn’t it an overkill - to have two caches in series?