ASM (Automatic storage Management)

ASM (Automatic storage Management)
ASM is storagement for Oracle Data.ASM is an extension to the OMF(Oracle Managed Files) it not only manages the storage automatically(reducing DBA job and time) it does the mirroring as well.

ASM is controlled by ASM instance found on each node.

Benefits of ASM:
• You don’t need to assign a layout for the Data, you just assign DISKS and that’s all.
• ASM will manage thousands of files automatically.
• It assigns the DISK groups to the DISKS.
• It is almost a Logical Volume Manager.
• It has built in mirring and stripping techniques.
• It creates and manages files automatically
• It automatically balances the IO balance between DISK groups.
• Manages Oracle files without specifying file and directory names.


What is ASM Database:
• When ASM is enabled, ORACLE will automatically create a small database where it will keep the metadata about ASM.
• The default Table Space created in ASM database is +ASM
• The default RAC nodes will appear as +ASMn
• ASM can be managed with srvctl commands


What are ASM Disks:
Disks assigned to ASM DISK groups are called ASM DISKS.
Files which are written to ASM DISKS are called ASM Files.

REDUNDANCY Groups for ASM DISK GROUPS:
There are three types of ASM Redundancies.
1.Normal Redundancy
2.High Redundancy
3.External Redundancy

1.Normal Redundancy:
Oracle will create a mirror DISK for each DISK.

2.High REDUNDANCY:
Oracle will create two mirror DISKS for each DISK.

3.External Redundancy
External Redundancy means that you will use some external hardware or tool for DISK mirroring. ORACLE wont do it for you.

Types of Files stored in ASM:
Data Files, Control Files, DataPump Dumpsets, table spaces, data guard files, spfiles.
Oracle Binaries, trace files and OS files cannot be stored in ASM.


ASM Instance and its Creation:
To create ASM instance you need to create a pramater file in /tmp directory.
Contents of SPfile would be
Instance_type=ASM


Export ORACLE_SID=+ASM

Creating SPFILE using ASM parameter file:
Sqlplus “/ as sysdba”
Create SPFile from – ASM ParameterFile—

Start the ASM instance with nomount mode:
Startup nomount

Now you can add DISK Groups and files.

You can use the Create DISKGroup command to create diskgroups.in this command you have to define the type of redundancy as well.
Note: you can alter,resize,drop,undrop DISGROUPS and files through SQL files.