I need to do maintenance on the SAN. Do I need to shutdown AIX as well? Or I can just unmount the SAN-FS and let the SAN reboot or do maintenance mode. And then when SAN is up, just mount it back.
It will depend on whether the system boots from the SAN.
If the system boots from local disks you should be able to unmount the file systems and vary off the volume groups, then do your SAN work, then import the VG, mount FS if required.
If the system boots from the SAN - rootvg on SAN disks - then you will need to power down the system / LPAR first.