(This is my first posting, please excuse any mistakes :-))

AIX provides a hierarchy scheme for storing data on DASD.  The highest 
group of data is something called a Volume Group.  A volume group is
a collection of one or more Logical Volumes.  Logical volumes are created
by telling AIX how many logical partitions you want to make up the logical
volume.  If you are using AIX defaults, the size of the logical partition
is 4Mb.  When a logical volume is created, it is usually given a name like
/dev/lv01 which can then be used to create a file system and mount at your
favorite mount point.  Logical volumes can be extended in increments of
logical partitions until you run out of DASD space.  The reverse is 
obviously not true - to shrink a volume group, you must delete it and
recreate it.

In AIX, you also have things called Physical Volumes and Physical Partitions.
A physical volume is just another name for a physical disk drive.  A
physical partition is an actual physical partition on the disk surface.
Again, the AIX default for these is 4Mb.  The nice thing about AIX is that
your logical partitions in a logical volume can be spread over more than
one physical drive.  For instance, if you have a heavily used file system,
you can create a logical volume that spans two physical drives, thus 
"increasing" disk throughput by "reducing" the load of any one actuator.

Since striping was brought up, I thought I'd mention that striping is 
different than mirroring.  In a mirrored system, data is written on two
or more actuators, thus if a drive goes down, data can still be retrieved
from another drive without any down time.  In striping, a block of data is
split into two or more pieces and written by actuators on sectors in the
same position across disk(s) in the volume.  Checksum data is also stored,
thus if any one sector becomes unavailable, it can be easily created 
using the checksum data.  (See _DASD_IBM's_Direct_Access_Storage_Devices_
by Johnson and Johnson, p. 383f.  ISBN 0-07-032674-6.)

As the system administrator for our group's RS/6000, I must say that the
way AIX handles data storage is very nice!  I just put a new 2Gb disk online
and carved it up without breaking a sweat.  Extending logical volumes 
(and file systems) sure comes in handy when loading in large licensed
programs!

Hope this helps...

Rich Niedziela
University of Chicago, Department of Chemistry
5735 South Ellis Avenue
Chicago, IL   60637

email:  rfn1@quads.uchicago.edu -or- niedz@cloister.uchicago.edu

(sorry - no .signature yet!)