2. Automatic Storage Management (ASM) Concepts and Overview

Oracle ASM Concepts:

 

ASM uses disk groups to store datafiles; an ASM disk group is a collection of disks that ASM manages as a unit. Within a disk group, ASM exposes a file system interface for Oracle database files. The content of files that are stored in a disk group are evenly distributed, or striped, to eliminate hot spots and to provide uniform performance across the disks. The performance is comparable to the performance of raw devices.

 

You can add or remove disks from a disk group while a database continues to access files from the disk group. When you add or remove disks from a disk group, ASM automatically redistributes the file contents and eliminates the need for downtime when redistributing the content.

 

The ASM volume manager functionality provides flexible server-based mirroring options. The ASM normal and high redundancy disk groups enable two-way and three-way mirroring respectively.

 

 

Oracle ASM Instances:

 

An ASM instance has a System Global Area (SGA) and background processes that are similar to those of Oracle Database. ASM has a minimal performance effect on a server.  ASM instances mount disk groups to make ASM files available to database instances. ASM instances do not mount databases. ASM instances manage the metadata of the disk group and provide file layout information to the database instances. ASM and database instances require shared access to the disks in a disk group.

 

ASM metadata is the information that ASM uses to control a disk group and the metadata resides within the disk group. ASM metadata includes the following information:

 

• The disks that belong to a disk group.
• The amount of space that is available in a disk group.
• The filenames of the files in a disk group.
• The location of disk group datafile data extents.
• A redo log that records information about atomically changing data blocks.

 

Oracle ASM Disk:

 

An Oracle ASM disk is a storage device that is provisioned to an Oracle ASM disk group. An Oracle ASM disk can be a physical disk or partition, a Logical Unit Number (LUN) from a storage array, a logical volume, or a network-attached file.

 

ASM disks are the storage devices that are provisioned to ASM disk groups. Examples of ASM disks include:

 

• A disk or partition from a storage array
• An entire disk or the partitions of a disk
• Logical volumes
• Network-attached files (NFS)

 

Oracle ASM Disk Groups:

 

A pool of disks managed as a logical disk is called disk groups. An Oracle ASM disk group is a collection of Oracle ASM disks managed as a logical unit. The data structures in a disk group are self-contained and consume some disk space in a disk group.

 

Within a disk group, Oracle ASM exposes a file system interface for Oracle database files. The content of files that are stored in a disk group are evenly distributed, or striped, to eliminate hot spots and to provide uniform performance across the disks. The performance is comparable to the performance of raw devices.

 

List of Oracle ASM Files:

 

Files that are stored in ASM disk groups are called ASM files.

 

• Control files
• Datafiles, temporary datafiles, and datafile copies
• SPFILEs
• Online redo logs, archive logs, and Flashback logs
• RMAN backups
• Disaster recovery configurations
• Change tracking bitmaps
• Data Pump.

 

Allocation Units (AU):-

 

ASM introduces the concept of an allocation unit (AU), the smallest contiguous disk space allocated by the ASM. The typical value for an AU is 1MB and is not user configurable. ASM does not allow physical blocks to be split across allocation units.

 

Oracle ASM Instance Background Processes

 

There are at least two new background processes added for an ASM instance:

   – RBAL – coordinates rebalance activity for disk groups

   – ORB0, ORB1… – These perform the actual rebalance data extent movements.

 

 

Oracle ASM Disk Group Administration:

 

Disk groups are created using the CREATE DISKGROUP statement. This statement allows specification of 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.

 

Oracle ASM Functionality:

 

Striping

 

ASM stripes its files across all the disks that belong to a disk group. It remains unclear if it follows a strict RAID 3 fashion of striping or a variant of RAID 3 that facilitates easy addition and removal of disks to and from the disk group. Oracle Corporation recommends that all the disks that belong to a disk group have the same size, in which case each disk gets the same number of extents. However, if a DBA configures disks of different sizes, each disk might get a different number of extents — based upon the size of the disk. An allocation unit typically has a size of 1MB.

 

ASM stripes help make data more reliably available and more secure than in other Oracle storage implementations.

 

Redundancy

 

One can configure ASM diskgroups to have no redundancy (external), two-way mirroring (normal), or three-way mirroring (high). In the case of normal and high mirrors, good practice suggests having fail groups that talk to different controllers for performance and fail-safe reasons.

 

In the case of external redundancy, ASM does not do any software mirroring, but only stripes its files across all the disks that belong to the disk group that does external redundancy.

 

In the case of normal redundancy, ASM does two-way mirroring, meaning that ASM maintains two copies of the data through software mirroring. When querying for mirror information, DBAs will see two mirrors in this case.

 

In the case of high redundancy, ASM does three-way mirroring, maintaining three copies of the data through software mirroring. When querying for mirror information, DBAs will see three mirrors in this case.

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