An ASM file extent size is a multiple of AUs.
Now, Oracle recommends 4 megabytes (MB) for the allocation unit (AU) size for a disk group. The operating system I/O size should be set to the largest size possible. Benefits of 4 MB AU include:
■ Increased I/O throughput of the I/O subsystem
■ Reduced SGA size to manage the extent maps in the database instance.
■ Faster datafile initialization
■ Increased file size limits
■ Reduced database open time
Allocation size is determined by the AU_SIZE attribute of the CREATE DISKGROUP command for the disk group. Allowable values for AU_SIZE are 1M, 2M, 4M, 8M, 16M, 32M or 64M in size. The AU_SIZE cannot be changed after the disk group is created.
Created by Charles Kim, Oracle ACE Director