Improve system latency during harddisk access

[Matthias Reyelt]

Hi,

We have a cyclic test application running at a 50ms cycle running on a
Marvell OpenRD with 9.0 with an attached sata harddisk. 

We have observed that the system is sometimes blocked up to about 18ms by
accessing the harddisk. We have checked the ULE and 4BSD scheduler and
preemption, but without any significant difference. 

I actually do not really believe that this is somewhat ARM specific. 
Generally, harddisk I/O is critical for latency, but I've cross checked with a 
Linux installed OpenRD and had less impact of harddisk I/O.

Do you have any idea how to minimize the influence of harddisk I/O under 
FreeBSD?

Regards
Matthias