Automatic Storage Management (ASM)

Contents:- Automatic Storage Management Theatrical Concepts

Oracle ASM Storage Components

ASM Disks:-                             

ASM Disk is a basic storage devices which can be added to oracle ASM Disk Groups.

ASM Disk Group:           

ASM Disk Group is a collection of one or more disks.ASM Disk Group using Striping Technique and also provides Data Redundancy using Mirroring Algorithm.

ASM Allocation Units     

Is the Smallest units in ASM Disk. ASM Disk is divided into Several ASM Allocation Units. ASM Allocation Unit size can be selected at the time of Creation ASM Disk Group. Allocation unit size cannot be change after creation of ASM Disk Group. Large Allocation units provides high performance for data warehouse application.

ASM Extents                            

ASM Extents is a collection of one or more allocation units. It is row storage.

ASM Files

In Oracle ASM all database files are store in ASM Disk Group. Those files are called as ASM Files.Oracle  ASM Supports Database Related Files i.e Control File, Data Files, Redo log files, Archive Files, Backup Files, Flashback Files, parameter file, configuration files, change tracking bitmap files.

Oracle ASM File is completely stored in one disk group.

ACFS 

ASM Cluster File System Can store non-database files.

ASM File Extent Map

ASM File Extent Map is a list of Extent Points pointed to already Extents. Each Extent point is describing details of Allocation unit number, When an Oracle Database send request to open ASM File at that time ASM Provides ASM File 

Extent Map to the database instance. Database Instance keeps this ASM File Extent Map in Memory and according to that Database Instance Perform I/O to ASM File and ASM Disks.

ASM Instance not mount or open an database.

Features

Oracle-ASM Variable size Extents:     (DG.Compatible.RDBMS>=11)

Variable size Extents Feature Handling Big File more efficiently.

Notes:- When New ASM File is Created Extents are allocated by using following Logic

First 20000 Extents are  of DG AU Size.

Next 20000 Extents are of 4 Times AU Size.

Above 40000 Extents are of 16 Times AU Size.

Oracle-RDBMS Connection

During Oracle Database Instance and ASM Instance Connections  any operations i.e create ASM file, open ASM file, Delete ASM file there is one COD (Continuing operation directory) is created by ASM Instance.

ASM Striping

Whenever database create any ASM File at that time ASM File is divided into Stripe Set. And then each ASM disk will store one Stripe from the stripe set and Striping continuous repeated until last stripe set. Oracle ASM Stripe provides load balancing across all disk groups. Striping also reduces I/O Latency.

Oracle ASM Failure Group

Oracle ASM Failure Group is a subset of Disks in a Disk Group. Failure Group are used to store mirror copies of data. Each Mirror Copy of data are store in a disk in a different Failure Group. Each disk in a Disk can belong to only one failure group. You can define a Failure group during ASM Disk group creation.

ASM Disk Group Rebalance Operation

RBAL Background process co-ordinates Rebalance Activity. ARBn processes performs actual rebalance operation.

During Rebalance operation ASM Creates COD Entry to track Disk Group Rebalance Operation.

You can set ASM_POWER_LIMIT Parameter to perform rebalance operation.

V$ASM_OPERATION view provides progress for rebalance operation.

Dynamic Performance Views ASM

V$ASM_DISK, V$asm_diskgroup, V$asm_alias

v$asm_client, V$asm_template, V$asm_operation, V$asm_file