[PLUG] Disk IO in Linux?

[Alex Daniloff]

Hello Linux folkz,
I'm sorry for these strange questions

After reading 
"Web Performance Tuning"
2nd Edit. 2002 by Patrick Killelea 
Pub.O'Reily. ISBN 0-598-00172-x  

I have the following assumptions:

Linux kernel uses buffered IO file access 
when it's reading and writing to the physical 
partition/device with its native FS.

Linux kernel uses unbuffered IO file access 
if a program (e.g. DB engine) reads and writes 
to the raw partition/device.
I assume it's because DB engine handles its
data transaction on raw device/partition 
without going through the kernel IO calls.
 
Does all above apply to the case if drives 
support DMA?

Why it's nessesary to bind raw devices to block devices
( bind /dev/raw/raw1 to /dev/hdb1 )
if a database engine reads and writes to the raw partition
without such binding?

Will this binding improve performance of the DB engine or
this needs to be done only in order to read from Linux what
has been written on a raw device?

Thank you in advance for any answers or sources of further
information.

Alex