Administering ASM Disk Groups

Disks

Disk groups are created using the CREATE DISKGROUP statement. This statement allows you to specify the level of redundancy:

• NORMAL REDUNDANCY - Two-way mirroring, requiring two failure groups.
• HIGH REDUNDANCY - Three-way mirroring, requiring three failure groups.
• EXTERNAL REDUNDANCY - No mirroring for disks that are already protected using hardware mirroring or RAID. If you have hardware RAID it should be used in preference to ASM redundancy, so this will be the standard option for most installations.

In addition failure groups and preferred names for disks can be defined. If the NAME clause is omitted the disks are given a system generated name like "disk_group_1_0001". The FORCE option can be used to move a disk from another disk group into this one.

CREATE DISKGROUP disk_group_1 NORMAL REDUNDANCY

  FAILGROUP failure_group_1 DISK

    '/devices/diska1' NAME diska1,

    '/devices/diska2' NAME diska2

  FAILGROUP failure_group_2 DISK

    '/devices/diskb1' NAME diskb1,

    '/devices/diskb2' NAME diskb2;

Disk groups can be deleted using the DROP DISKGROUP statement.

DROP DISKGROUP disk_group_1 INCLUDING CONTENTS;

Disks can be added or removed from disk groups using the ALTER DISKGROUP statement. Remember that the wildcard "*" can be used to reference disks so long as the resulting string does not match a disk already used by an existing disk group.

-- Add disks.

ALTER DISKGROUP disk_group_1 ADD DISK

  '/devices/disk*3',

  '/devices/disk*4';

-- Drop a disk.

ALTER DISKGROUP disk_group_1 DROP DISK diska2;

Disks can be resized using the RESIZE clause of the ALTER DISKGROUP statement. The statement can be used to resize individual disks, all disks in a failure group or all disks in the disk group. If the SIZE clause is omitted the disks are resized to the size of the disk returned by the OS.

-- Resize a specific disk.

ALTER DISKGROUP disk_group_1

  RESIZE DISK diska1 SIZE 100G;

-- Resize all disks in a failure group.

ALTER DISKGROUP disk_group_1

  RESIZE DISKS IN FAILGROUP failure_group_1 SIZE 100G;

-- Resize all disks in a disk group.

ALTER DISKGROUP disk_group_1

  RESIZE ALL SIZE 100G;

The UNDROP DISKS clause of the ALTER DISKGROUP statement allows pending disk drops to be undone. It will not revert drops that have completed, or disk drops associated with the dropping of a disk group.

ALTER DISKGROUP disk_group_1 UNDROP DISKS;

Disk groups can be rebalanced manually using the REBALANCE clause of the ALTER DISKGROUP statement. If the POWER clause is omitted the ASM_POWER_LIMIT parameter value is used. Rebalancing is only needed when the speed of the automatic rebalancing is not appropriate.

ALTER DISKGROUP disk_group_1 REBALANCE POWER 5;

Disk groups are mounted at ASM instance startup and unmounted at ASM instance shutdown. Manual mounting and dismounting can be accomplished using the ALTER DISKGROUP statement as seen below.

ALTER DISKGROUP ALL DISMOUNT;

ALTER DISKGROUP ALL MOUNT;

ALTER DISKGROUP disk_group_1 DISMOUNT;

ALTER DISKGROUP disk_group_1 MOUNT;

ASM Views

The ASM configuration can be viewed using the V$ASM_% views, which often contain different information depending on whether they are queried from the ASM instance, or a dependant database instance.

View	ASM Instance	DB Instance
V$ASM_ALIAS	Displays a row for each alias present in every disk group mounted by the ASM instance.	Returns no rows
V$ASM_CLIENT	Displays a row for each database instance using a disk group managed by the ASM instance.	Displays a row for the ASM instance if the database has open ASM files.
V$ASM_DISK	Displays a row for each disk discovered by the ASM instance, including disks which are not part of any disk group.	Displays a row for each disk in disk groups in use by the database instance.
V$ASM_DISKGROUP	Displays a row for each disk group discovered by the ASM instance.	Displays a row for each disk group mounted by the local ASM instance.
V$ASM_FILE	Displays a row for each file for each disk group mounted by the ASM instance.	Displays no rows.
V$ASM_OPERATION	Displays a row for each file for each long running operation executing in the ASM instance.	Displays no rows.
V$ASM_TEMPLATE	Displays a row for each template present in each disk group mounted by the ASM instance.	Displays a row for each template present in each disk group mounted by the ASM instance with which the database instance communicates.