I was checking my system this morning, having it booted for the first time after the installation of Oracle 11g Release 2 Grid Infrastructure.
I opened up SQL*plus for a connection to ASM, to see what the disk space usage was.
I found the following information:
The first section of the screen shows the total amount of diskspace and available diskspace from an ASM perspective, the lower part of the screen shows the file system usage.
Here you can see that the file system, even though it has practically all space available, has claimed all of its space in ASM. Logically, you might think, but since the two are so tightly integrated, might surprise a bit.
The principle is the same compared to 10g ASM with Oracle Datafiles. The datafile itself might not been occupied fully, still all of the space that the datafile is configured with needs to be allocated in ASM. When you want to see what your actual usage is on an ASM disk and how much you can grow, you would need to calculate the free space in your file system and add the FREE_MB column from within ASM to it to determine the exact free space on your ASM Disk(s).