Database Software

Hello all,
I'm trying to learn a bit regarding ASM and the best practices to follow to properly configure/use it at its best.

Using an external FC SAN storage, if I create a logical volume on it assigning a LUN number, and expose this LUN to my ASM server, I can correctly see it and add it to an ASM disk group.

So far so good. After that, if I expand this LUN on the external storage (using the appropriate tools), I'm not able to see the new allocated space from ASM. Looks like Oracle somehow originally "partitioned" the LUN when I first added it to the ASM instance, and after "tagging" it, even if I expand the Volume, the "expansion" is not seen.

Is there a way to tell ASM that ther actually IS more space available? If I "fdisk -l", I see the correct amount of space on the device. But the "blocks" are still the old value....

I know that creating another LUN, and adding it to the Disk Group, there wouldn't be any problems, but I'd rather like to be able to expand existing LUNs on my external storage than creating new LUNs on it all the times... Please help me to understand if what I have in mind is correct or not on a best practices point of view.

Thank you for any help.