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25 Jul, 2025
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ASM overview and Architecture

1 . ASM in Oracle RAC

In an Oracle RAC environment, ASM plays a critical role in managing the shared storage that all nodes in the cluster access. Each node in the Oracle RAC environment uses ASM to manage the database files stored on shared storage, ensuring that all nodes have access to the same data.

2 . ASM Architecture

ASM architecture consists of the following key components:

ASM Instances

Each Oracle RAC node has its own ASM instance. The ASM instances coordinate with each other to manage the shared storage.
ASM instances are lightweight and manage disk groups rather than handling database operations directly. These instances communicate with the database instances running on the same node to provide storage management services.

Disk Groups

Disk Groups: A disk group is a logical collection of disks managed by ASM. Disks in a disk group are automatically striped and mirrored to balance the load and ensure data redundancy.

ASM distributes database files across all disks in a disk group, optimizing performance and ensuring high availability. Disk groups can include data files, control files, log files, and other Oracle database-related files.

There are three types of ASM redundancy levels:

External Redundancy: No mirroring; data redundancy is managed by an external storage array.
Normal Redundancy: Provides 2-way mirroring, protecting against the failure of a single disk.
High Redundancy: Provides 3-way mirroring, protecting against the failure of two disks.

ASM Disk

Disks in ASM can be physical disks, LUNs (logical unit numbers), or files representing disks (e.g., in a test environment).

ASM manages disks by assigning them to disk groups, where the disk space is pooled and used for storing data.

ASM Allocation Units (AU)

ASM divides each disk into allocation units (AU), which are the smallest unit of storage managed by ASM. By default, an allocation unit is 1 MB in size.

When a file is created in ASM, it is divided into extents, which are stored in allocation units across the disks in the disk group. ASM automatically manages the placement and distribution of these extents to balance I/O loads and optimize performance.

ASM Extents

ASM breaks files into smaller pieces called extents, which are spread across multiple disks in the disk group. This striping improves I/O performance by allowing parallel access to data.

The extent size can vary based on the file size and can automatically grow as the file size increases.

ASM Striping

ASM uses striping to evenly distribute database files across all disks in a disk group, improving performance by spreading I/O operations across multiple devices. There are two types of striping in ASM:

Coarse Striping: Distributes data across multiple disks at a larger granularity, typically used for larger I/O operations (e.g., data files).
Fine Striping: Distributes data at a smaller granularity, typically used for smaller I/O operations (e.g., redo log files).

ASM Mirroring and Redundancy

ASM provides built-in redundancy options to protect against disk failures. The mirroring is managed automatically by ASM and is configurable at the disk group level.

Normal Redundancy (2-way Mirroring): Each extent is mirrored on two different disks, providing protection against a single disk failure.

High Redundancy (3-way Mirroring): Each extent is mirrored on three different disks, providing protection against two simultaneous disk failures.

External Redundancy: No mirroring is performed by ASM, and redundancy is managed externally by the underlying storage system.

Rebalancing in ASM

One of the most powerful features of ASM is its ability to automatically rebalance data when disks are added or removed from a disk group. This ensures that data remains evenly distributed across all disks, preventing hotspots and optimizing performance.

Automatic Rebalancing: When a new disk is added to a disk group, ASM automatically redistributes the data to include the new disk, without any manual intervention.

Controlled Impact: The rebalance operation is done with minimal impact on database performance. You can control the speed of the rebalancing operation by setting the ASM_POWER_LIMIT parameter.

ASM and ACFS (Oracle ASM Cluster File System)

ASM also supports Oracle ASM Cluster File System (ACFS), which extends ASM functionality to manage general-purpose file systems, not just database files.

ACFS provides a scalable, clustered file system solution that integrates with ASM for features like mirroring, striping, and snapshot management.

Dynamic Disk Group Changes

Adding Disks: You can add disks to a disk group at any time, and ASM will automatically rebalance the data across the new and existing disks.

Dropping Disks: Similarly, disks can be dropped from a disk group, and ASM will automatically move the data off the disk being removed.

This flexibility allows for non-disruptive changes to the storage configuration, ensuring high availability and scalability in RAC environments.

ASM and Oracle RAC Integration

ASM is fully integrated with Oracle RAC, providing a unified storage management layer for the entire cluster. Each Oracle RAC node accesses the shared storage through ASM, which ensures that all database files are consistently available across all nodes.

Key points of integration:

Shared Access: All Oracle RAC instances access the same database files through ASM, ensuring data consistency.

Automatic Failover: If a node fails, the remaining nodes in the RAC cluster continue to access the shared storage through ASM, ensuring continuous availability.

Scalability: ASM’s ability to dynamically manage storage allows Oracle RAC to scale seamlessly as more storage or nodes are added to the cluster.

ASM Tools and Utilities

asmcmd: A command-line utility that allows you to manage ASM instances and disk groups. You can use it to perform tasks like adding or removing disks, creating or dropping disk groups, and monitoring ASM status.

Enterprise Manager (OEM): Oracle Enterprise Manager provides a graphical interface for managing ASM, including monitoring and administration tasks.