Hello,
I've been reading the AIX 5.1 System Concepts PDF but I'm still confused about some file-system concepts and their relationships.
What is the 'allocation group size'?
How are 'disk allocation unit'/'fragment size', NBPI and 'allocation group size'?
And how are the above three concepts realted logical parition size?
Thanks
I don't know aix, but I took a quick look at some web sites and it looks like aix is using an upgraded version of the McKusick filesystem. So I'll take a quick stab at some of this...
NBPI
An inode is a thing that you need to create a file. If a filesystem has 1000 inodes, then it can have 1,000 files. You want to have too many inodes. When you run out of disk space, you will have some unused inodes. That is better than unused disk space with no inodes left to use it. NBPI is number of bytes per inode. Think of it as the average expected file size. By having a default expressed this way, if the filesystem doubles, so do the inodes.
Allocation Group
A filesystem is big so it get chopped up into a few sections called allocation groups. Think about a file that is growing...it needs another data block. You don't want any data block, you want one close by. So the system tries to keep the file in the same allocation group. In fact it tries to keep all of the files in a directory (not subdirectory) in the same allocation unit. Allocation units should be large enough for this to be possible most of the time.
allocation unit
This is the smallest piece of data that can be assigned to a file. A one byte file will use one allocation unit.
That's a quick rundown of these terms. But if you don't understand them all, just take the default values for everything.
Note to freaked out file system gurus...I know that I'm over-simplifying. I think that's what the OP wanted.
From IBM Redbook website you could find book called AIX Logical Volume Manager from A to Z: Introduction and Concepts. It might help you to figure out LVM concept in AIX.
At least it did so to me.. 
--Tommy