This happens in the background while the database objects contained
in the disk files are still online and available to users. If the impact to the I/O subsystem is high
during a rebalance operation, the speed at which the rebalance occurs can be reduced using an
initialization parameter.
Chapter 4: Physical Database Layouts and Storage Management 99
ASM requires a special type of Oracle instance to provide the interface between a traditional
Oracle instance and the file system; the ASM software components are shipped with the Oracle
database software and are always available as a selection when you??™re selecting the storage type
for the SYSTEM, SYSAUX, and other tablespaces when the database is created.
Using ASM does not, however, prevent you from mixing ASM disk groups with manual Oracle
datafile management techniques such as those I presented in Chapter 3 and earlier in this chapter.
However, the ease of use and performance of ASM makes a strong case for eventually using ASM
disk groups for all your storage needs.
Two Oracle background processes introduced in Oracle Database 10g support ASM instances:
RBAL and ORBn. RBAL coordinates the disk activity for disk groups, whereas ORBn, where n can
be a number from 0 to 9, performs the actual extent movement between disks in the disk groups.
For databases that use ASM disks, there are also two new background processes as of Oracle
Database 10g: OSMB and RBAL. OSMB performs the communication between the database and
the ASM instance, whereas RBAL performs the opening and closing of the disks in the disk group
on behalf of the database.
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