Write cache for pst (Promise SX6000 RAID)?

[ZC Wong]

I've got a Promise SX6000 RAID card and the pst driver works without any
configuration under a GENERIC kernel came with 6.0-RELEASE.

The problem is the write caching at OS level, it doesn't look like any data
is cached by the OS before it's written (even if there's plenty of free
mem).

Is there any chance I can get the pst driver work like /dev/ad* in term of
caching?

Also there seemed to be a bottleneck of 50MB/s whatever how much hard drives
I connect to the array(RAID 0)?

Regards,

Zi