Chapter 12 ■ tuning the Storage SubSyStem
During the RDBMS instance startup process, this information is used by the RDBMS instance to retrieve all diskgroup information from the ASM instance.
4. When a user adds a new disk to an existing diskgroup, the RBAL background process will create a map to reorganize the extents in the diskgroup. The RBAL process will then send a message to the ARBn background process to reorganize the extents. The number of ARBn background processes started is based on the parameter ASM_POWER_LIMIT.
a. The DBA issues a statement to create a tablespace or add a datafile; the database requests ASM for file creation.
b. An ASM foreground process creates a Continuing Operation Directory (COD) entry and allocates space for the new file across the diskgroup.
9. Based on user activity on the database instance, any updates to the data on the ASM devices are performed by the DBWR process on the RDBMS instance. Such activity is performed using the layout obtained by the RDBMS instance from the ASM instance (illustrated in Step 6 above).
•� 16 AUs for extents above 40,000
For the first 20,000 extents within a file, each extent contains one allocation unit. Extents are allocated on different disks in a round robin fashion in order to spread the file and distribute I/O during read/write operations. Beyond the first 20,000 extents, space is allocated in groups of eight extents at a time. Each extent contains four allocation units for the second 20,000 extents, or 16 allocation units for all extents beyond 40,000. Extents are still allocated on different disks to spread the file out, but, in addition, each allocation unit is written in turn to a different extent until each set of eight extents is filled. The result is that consecutive allocation units will always be in different extents.