Introduction to Oracle ASM
Oracle Automatic Storage Management (ASM) is a volume manager and file system designed specifically for Oracle databases. Its main goal is simple:
Let Oracle manage database storage the same way it manages data.
Instead of DBAs manually managing files, disks, and I/O balancing, ASM automates these tasks, improves performance, and simplifies storage administration.
Why Oracle ASM Is Needed
Traditional file systems require DBAs to:
Manually place datafiles on disks Balance I/O across disks Handle disk failures and mirroring Resize storage manually
Oracle ASM removes this complexity by providing database-aware storage intelligence.
Core Components of Oracle ASM
1. ASM Instance
The ASM instance is a lightweight Oracle instance responsible for storage management.
It does not contain user data.
Key responsibilities:
Manages ASM metadata Controls disk groups Handles rebalancing Maintains failure groups Runs ASM background processes (RBAL, ARBx)
Each database connects to the ASM instance to access storage.
2. ASM Disk Groups
An ASM Disk Group is a logical collection of disks or LUNs.
Common disk groups:
+DATA → Datafiles +FRA → Archive logs, backups +REDO → Redo logs
Disk groups abstract physical storage and provide:
Automatic striping Automatic mirroring Centralized storage management
3. Raw Disks / LUNs
ASM works directly on raw disks or LUNs, not traditional file systems.
These disks are:
Discovered by ASM Added to disk groups Used collectively for storage
ASM treats all disks equally, distributing data across them.
4. Extents – How ASM Stores Data
ASM breaks every database file into extents.
What are extents?
Small chunks of data Spread evenly across all disks in a disk group
Benefits:
Balanced I/O load Improved performance No hot disks
This is why ASM scales very well as disks are added.
5. ASM Metadata
ASM maintains its own metadata, which tracks:
File locations Extent maps Disk usage Redundancy information
This metadata allows ASM to:
Know where every extent is stored Rebuild data during failures Rebalance data automatically
6. Redundancy and Failure Groups
ASM provides built-in mirroring, eliminating the need for RAID (in many cases).
Redundancy levels:
External – No mirroring (storage handles it) Normal – Two-way mirroring High – Three-way mirroring
Failure Groups ensure mirrored copies are placed on separate disks or storage units, protecting against disk or controller failures.
7. Rebalancing – Automatic Storage Optimization
One of ASM’s most powerful features is rebalancing.
When you:
Add a disk Remove a disk Drop a disk
ASM automatically:
Redistributes extents Maintains uniform I/O Preserves redundancy
This happens online, without database downtime.
Files Managed by ASM
ASM manages all critical Oracle database files, including:
Datafiles Redo log files Control files Tempfiles Archive logs Backup pieces
DBAs reference files using ASM aliases like:
+DATA/orcl/datafile/system.256.123456789
Advantages of Oracle ASM
Simplified storage management Automatic I/O load balancing Built-in redundancy Online disk addition and removal High availability Better performance Tight integration with Oracle RAC
Oracle ASM in RAC Environments
ASM is mandatory for Oracle RAC in most production setups because:
All nodes share the same disk groups Storage consistency is maintained Failures are handled transparently
Conclusion
Oracle ASM is not just a storage option—it is a database-aware storage management system.
By understanding concepts like disk groups, extents, metadata, redundancy, and rebalancing, DBAs can design highly available, scalable, and performant Oracle databases with minimal effort.