oracle 11gR2 版本用asmca命令创建磁盘组时,可以选择 au_size 的大小,比如1M, 4M ,8M,16M
这样对支持大容量的数据库非常有好处 ,有以下5个好处：
■ Increased I/O through the I/O subsystem if the I/O size is increased to the AU
size.
■ Reduced SGA size to manage the extent maps in the database instance.
■ Faster datafile initialization if the I/O size is increased to the AU size.
■ Increased file size limits.
■ Reduced database open time.
The allocation unit size is specified with the disk group attribute AU_SIZE. The AU
size cannot be altered after a disk group is created. Example 4–1 on page 4-5 shows
how the AU_SIZE is specified with the CREATE DISKGROUP SQL statement.
For applications that can benefit from large I/O sizes, you can increase the
DB_FILE_MULTIBLOCK_READ_COUNT initialization parameter to 4 MB from the
default of 1 MB. Ensure that the application is thoroughly tested with this new setting
as this change could adversely affect stable optimized plans and future optimizer plan
generation.
To change the DB_FILE_MULTIBLOCK_READ_COUNT setting, use the ALTER SYSTEM
SQL statement, as shown in Example 4–3. This example assumes an 8 K block size.
Example 4–3 Altering the DB_FILE_MULTIBLOCK_READ_COUNT Setting
SQL> ALTER SYSTEM SET db_file_multiblock_read_count=512 scope=both sid=*
See Also: Oracle Database Reference for more information about the
DB_FILE_MULTIBLOCK_READ_COUNT initialization parameter
用asmca创建diskgroup时可以选择au_size大小，diskgroup一旦创建好后au_size不能修改。
而 oracle 10g au_size 默认大小为 1M，一般情况下是不能修改的，而如果在10g版本中要支持TB或PB的大数据量， 那如何修改，下篇文章会有介绍