In addition, I??™ll show how you create a special type of
Oracle instance to support ASM as well as how to start up and shut down an ASM instance. We??™ll
review the new initialization parameters related to ASM and the existing initialization parameters
that have new values to support an ASM instance. Also, I??™ll introduce the asmcmd command-line
utility, new to Oracle 10g Release 2, that gives you an alternate way to browse and maintain objects
in your ASM disk groups. Finally, I??™ll use some raw disk devices on a Linux server to demonstrate
how disk groups are created and maintained.
ASM Architecture
ASM divides the datafiles and other database structures into extents, and it divides the extents among
all the disks in the disk group to enhance both performance and reliability. Instead of mirroring
entire disk volumes, ASM mirrors the database objects to provide the flexibility to mirror or stripe
the database objects differently depending on their type. Optionally, the objects may not be striped
at all if the underlying disk hardware is already RAID enabled, part of a storage area network (SAN),
or part of a network-attached storage (NAS) device.
Automatic rebalancing is another key feature of ASM. When an increase in disk space is
needed, additional disk devices can be added to a disk group, and ASM moves a proportional
number of files from one or more existing disks to the new disks to maintain the overall I/O
balance across all disks.
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