The RESIZE clause of ALTER DISKGROUP enables you to resize disks in a disk group. All disks in the disk group must be of equal size.
If you do not specify a new size in the SIZE clause, then Oracle ASM uses the size of the disks as returned by the operating system. The new size is written to the Oracle ASM disk header and if the size is increasing, then the new space is immediately available for allocation. If the size is decreasing, rebalancing must relocate file extents beyond the new size limit to available space below the limit. If the rebalance operation can successfully relocate all extents, then the new size is made permanent, otherwise the rebalance fails.
The following example resizes all of the disks in the disk group data1. If the new size is greater than the capacity of a disk, then the statement fails.
Example 4-7 Resizing disks in disk groups
ALTER DISKGROUP data1 RESIZE ALL SIZE 100G;