Speedy Disk I/O

[Benson I. Margulies]

I've been reading about logical volumes. 

Say one was responsible for porting an application to AIX 3.1 that
needed to do fast disk I/O. Say that the application was accustomed,
on Other Unix Systems, to using the character device interface to a
disk partition to get optimal performance, and perhaps even knew how
to read tracks or cylinders. (not so easy on a SCSI disk, but let that
pass).

Now, on AIX, one has two choices. If one can monopolize an entire
disk, one can use the hdisk device, and proceed as in the past.
An entire disk is a lot. To get less than a disk, one must take 
a logical volume.

It appears that there is no such thing as a contiguous logical volume,
so any track and cylinder cleverness would be right out, right?
A logical volume can span multiple physical volumes. A Smart Program
would try to take advantage of this by overlapping seeks between
the disks. Is there as way to know which PP's are on which disk?

Logical volumes can expand. Would a polite data storage program grow
logical volumes automatically, or provide out-of-space errors until
the administrator chlv'd some more space into existence?

Has anyone out there any useful experience with all of this?

-- 
Benson I. Margulies