1. some little words about the space allocation in VG/LV
When you create a Volume Group (VG), you specify the Physical Extent Size (PE). This depends upon the size of the inode-table you want + the total size of the VG.
You can make Logical Volumes in this VG with size multiple PE.
I don't think this is the real AIX-block size, but it's a real 'limit' you have to be aware of when creating VG and LV. eg. when you PE-size = 4M it's not possible to have a LV that is 6M large. it will have be 4M or 8M
2. AIX block size
I think the REAL AIX block size is 512 bytes.
If you do a df, AIX will report all sizes in 512bytes blocks.
If you do a df -k, AIX will report all size in Kbytes.
3. JFS cache
With regards to fast in/output, don't forget that AIX uses a lot of JFS cache.
If you want, you can tune this using vmtune ( /usr/samples/kernel ) with the -p and -P options. This can help you reducing the JFSCache thus resulting in more memory that can be used the db.