Managing Diskgroups

Managing Diskgroups

Creating Diskgroup
1. Assign Unqiue name for each diskgroup
2. Specify redundancy level of disk group

There is three type of redundancy level
1. NORMAL redundancy with 2 way mirroring
2. HIGH redundancy with 3 way mirroring
3. EXTERNAL redundancy with NO mirroring.

3. Specify disks as belonging to specific failgroup.
If we manually not define any failgroup for disk then Oracle automatically create failgroup for each diskgroup.

4. Specify asm disks (v$asm_disk) for diskgroup.

5. Optionally specify disk group attributes such as compatibility or allocation unit size.

SQL> conn / as sysasm
Connected to an idle instance.
SQL> startup nomount
ASM instance started
Total System Global Area 267825152 bytes
Fixed Size 1332584 bytes
Variable Size 241326744 bytes
ASM Cache 25165824 bytes

Create DISKGROUP with NORMAL REDUNDANCY
SQL> CREATE DISKGROUP dgrp1 NORMAL REDUNDANCY
2 FAILGROUP controller1 DISK
3 'e:\asmdisks\disk1' NAME diska1,
4 'e:\asmdisks\disk2' NAME diska2
5 FAILGROUP controller2 DISK
6 'e:\asmdisks\disk3' NAME diska3;
Diskgroup created.

SQL> select group_number, name, type, total_mb, free_mb from v$asm_diskgroup;
GROUP_NUMBER NAME TYPE TOTAL_MB FREE_MB
------------ ------------------------------ ------ ---------- ------
1 DGRP1 NORMAL 1536 1432

ADD NEW DISK in exiting DISKGROUP dgrp1
SQL> alter diskgroup dgrp1 add disk 'e:\asmdisks\disk4';
Diskgroup altered.

SQL> select group_number, name, type, total_mb, free_mb from v$asm_diskgroup;
GROUP_NUMBER NAME TYPE TOTAL_MB FREE_MB
------------ ------------------------------ ------ ---------- ------
1 DGRP1 NORMAL 2048 1940

Managing Capacity in Disk Groups
When ASM provides redundancy, such as when you create a disk group with NORMAL or HIGH redundancy, you must have sufficient capacity in each disk group to manage a re-creation of data that is lost after a failure of one or two failure groups. After one or more disks fail, the process of restoring redundancy for all data requires space from the surviving disks in the disk group. If not enough space remains, then some files might end up with reduced redundancy.

Negative Values of USABLE_FILE_MB
Due to the relationship between FREE_MB, REQUIRED_MIRROR_FREE_MB, and USABLE_FILE_MB, USABLE_FILE_MB can become negative. Although this is not necessarily a critical situation, it does mean that:
Depending on the value of FREE_MB, you may not be able to create new files.

The next failure might result in files with reduced redundancy.

If USABLE_FILE_MB becomes negative, it is strongly recommended that you add more space to the disk group as soon as possible.

When Should External, Normal, or High Redundancy Be Used?
ASM mirroring runs on the database server and Oracle recommends to off load this processing to the storage hardware RAID controller by using external redundancy. You can use normal redundancy in the following scenarios:

Storage system does not have RAID controller
Mirroring across storage arrays
Extended cluster configurations

In general, ASM mirroring is the Oracle alternative to third party logical volume managers. ASM mirroring eliminates the need to deploy additional layers of software complexity in your Oracle database environment.