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Oracle Managing ASM Disk Groups

Oracle Tips by Burleson Consulting

This is an excerpt from "Oracle 10g New Features for Administrators" by Ahmed Baraka.

ASM Striping

• For performance reasons, you must use disks of the same type and performance   capacity in a disk group.

• ASM provides two types of data striping, depending on the database file type:

Coarse striping: The stripe size is a relatively large 1MB chunk of file space. You may use coarse striping for all files in an Oracle database, except the control files, online redo log files, and flashback files.

Fine striping To reduce file latency, ASM provides a fine striping scheme, where the striping is in smaller chunk sizes of 128KB. You may want to use fine striping for control files, online redo log files, and flashback files.

ASM Mirroring

Disk mirroring provides data redundancy. If you lose a disk, you can use its mirror disk to continue operations without missing a beat. ASM mirrors extents.

Failure Groups

Failure groups define disks that share components, such that if one fails then other disks sharing the component might also fail.

Types of ASM Mirroring

• External redundancy You choose this level of mirroring when you are using operating   system storage array protection. Disk groups under this redundancy level don’t have any   failure groups.

• Normal redundancy This type provides two-way mirroring. Thus, to support a normal   redundancy level, you must create at least two failure groups.

• High redundancy This type provides three-way mirroring. You must create at least   three failure groups.

Creating a Disk Group

SQL> STARTUP NOMOUNT
SQL> CREATE DISKGROUP dgroup1 NORMAL REDUNDANCY
FAILGROUP controller1 DISK
'/devices/diska1' name testdisk size 100G,
'/devices/diska2',
'/devices/diska3'
FAILGROUP controller2 DISK
'/devices/diskb1',
'/devices/diskb2',
'/devices/diskb3'

You can force a disk that is already a member of another disk group to become a member of the disk group you are creating by specifying the FORCE

Note: The CREATE DISKGROUP statement mounts the disk group for the first time, and adds the disk group name to the ASM_DISKGROUPS initialization parameter if a spfile is being used. If a pfile is being used and you want the disk group to be automatically mounted at instance startup, then you must add the disk group name to the ASM_DISKGROUPS initialization parameter before the next time that you shut down and restart the ASM instance.

Adding Disks to a Disk Group

ALTER DISKGROUP dgroup1 ADD DISK
'/devices/diska5' NAME diska5,
'/devices/diska6' NAME diska6;
ALTER DISKGROUP dgroup1 ADD DISK
'/devices/diska*';

• When a disk is added, it is formatted and then rebalanced.

• When you don’t specify a FAILGROUP clause, the disk is in its own failure group.

• If you don't specify the NAME clause, Oracle assigns its own system-generated names.

• If the disk already belongs to a disk group, the statement will fail.

• Use the FORCE clause to add a disk that is a current member of disk group.

Dropping Disks and Disk Groups

ALTER DISKGROUP dgroup1 DROP DISK diska5;
DROP DISKGROUP test_groupa INCLUDING CONTENTS;

• DROP DISKGROUP statements requires the instance to be in MOUNT state.

• When a disk is dropped, the disk group is rebalanced by moving all of the file extents   from the dropped disk to other disks in the disk group. The header on the dropped disk is   then cleared.

• If you specify the FORCE clause for the drop operation, the disk is dropped even if   Automatic Storage Management cannot read or write to the disk.

• You can also drop all of the disks in specified failure groups using the DROP DISKS IN   FAILGROUP clause.

Undropping Disks in Disk Groups

ALTER DISKGROUP dgroup1 UNDROP DISKS;

• This statement enables you to cancel all pending drops of disks within disk groups.

Rebalancing Disk Groups

You can increase the speed of a rebalancing operation by doing any of the following things:

o raising the value of the ASM_POWER_LIMIT initialization parameter
o using a high value for the POWER clause in a disk rebalance operation

ALTER DISKGROUP dgroup1 REBALANCE POWER 5

o performing all your disk adding, resizing, and dropping operations at the    same time.

 
If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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