Database Reference
In-Depth Information
3.
High Redundancy
: Oracle ASM uses three-way mirroring. It requires at least three failure
groups for mirroring. Due to this three-way mirroring, the effective disk space is one-
third of all the disk capacity. With this setting, the ASM diskgroup can tolerate the failure
of two failure groups: if the primary copy of the extent fails, one of the mirroring copies
will be used. If both the primary copy and one of the mirroring copies fail, the remaining
mirroring copy will be used.
The ASM diskgroup can be created with the ASMCA GUI tool, or with a SQL command like this one after login to
ASM instance using SYSASM privilege:
SQL> CREATE DISKGROUP data NORMAL REDUNDANCY
FAILGROUP fg1 disk 'ORCL:DATA1' name data1, 'ORCL:DATA2' name data2,
FAILGROUP fg2 disk 'ORCL:DATA3' name data3, 'ORCL:DATA4' name data4;
If you decide to take advantage of the RAID 1+1 configuration in external SAN storage, you can use external
redundancy for the diskgroup:
SQL> CREATE DISKGROUP data EXTERNAL REDUNDANCY
Disk 'ORCL:DATA1' name data1, 'ORCL:DATA2' name data2;
You can perform the diskgroup administration tasks in an ASM instance on one of the RAC nodes with sqlplus:
SQL> Drop DISKGROUP data INCLUDING CONTENTS;
SQL> ALTER DISKGROUP DATA ADD DISK 'OCR:DATA3' REBALANCE Power 3
SQL> ALTER DISKGROUP DATA DROP DISK data2 REBALANCE Power 3
The REBALANCE Power clause specifies the power for the rebalancing operation. Combining the add disk and
drop disk operations in a single alter diskgroup operation will allow you to perform online storage migration for your
database: that is, to migrate your database from one storage to another while keeping it online.
For example, imagine that you want to migrate your database from an old storage to a new storage. DATA1 and
DATA2 ASM disks are on the old storage and DATA3 and DATA4 ASM disks are on the new storage. Executing the
following 'alter diskgroup' SQL command in the ASM instance of one RAC node will migrate your database from the
old storage to the new storage without any database downtime:
SQL>ALTER DISKGROUP DATA ADD DISK 'ORCL:DATA3' , 'ORCL:DATA4'
DROP DISK 'ORCL:DATA1','ORCL:DATA2' REBALANCE Power 8;
This feature has been widely used for storage migration and can be considered one of the greatest benefits of
using ASM as the storage solution for Oracle Database.
The range of values for Power clause is 0-11 inclusive, if the diskgroup ASM compatibility is set to less than
11.2.0.2. This range is extended to 1-1024 if the diskgroup ASM compatibility is set to 11.2.0.0 or higher; for example,
COMPATIBLE.ASM=12.1.0.0 in Oracle 12cR1 ASM.ASMCMD Utility and File System.
Oracle ASM provides the volume manager and a file system for the Oracle Database files.
When an ASM diskgroup is created in ASM, this diskgroup will be presented as a file system for Oracle Databases.
Unlike the OS file system, which you see and manage through OS file system commands, ASM files have to be
managed through the following two interfaces:
•
Oracle ASM command-line utility (ASMCMD)
: This utility provides a tool to administer Oracle
ASM; for example, to manage ASM instances, diskgroups, and file access control for disk
groups, files, and directories with a diskgroup.
•
SQL commands
: You can log in to the ASM instance to execute the SQL command. It also
provides a set of V$ ASM views for you to query the status of the ASM disks, diskgroup, etc.
