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ASM

Architecture

What is Automatic Storage Management

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Automatic storage management (ASM) is a new feature in Oracle Database 10g from oracle . It integrates file system and the Logical Volume Manager (LVM) . In ASM Volume Manager is specifically built for Oracle database files. The ASM can provide management for single SMP machines or across multiple nodes of a cluster for Oracle Real Application Clusters support.

Automatic Storage Management (ASM) simplifies administration of Oracle related files by allowing the administrator to reference disk groups rather than individual disks and files, which are managed by ASM.

Manual I/O tuning can be eliminated while ASM distributes input/output (I/O) load across all available resources to optimize performance while removing the need for manual I/O tuning.

The ASM has the flexibility of maintaining redundant copies of data to provide fault tolerance, or it can be built on top of vendor-supplied reliable storage mechanisms. Data management in ASM is basically done by choosing the desired reliability and performance characteristics for classes of data rather than with human interaction of per-file basis.

Automated storage management gives the time to DBA’s by increasing their ability to manage larger databases and more of them with increased efficiency.

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Automatic Storage Management (ASM) is a feature in Oracle Database 10g/11g that provides the database administrator with a simple storage management interface that is consistent across all server and storage platforms. ASM provides the performance of async I/O with the easy management of a file system.

Some of the Key features of ASM

  • Stripes files rather than logical volumes
  • Enables online disk reconfiguration and dynamic rebalancing
  • Provides adjustable rebalancing speed
  • Provides file based redundancy
  • Supports only Oracle files
  • It’s cluster aware

Why ASM ?

Some of the storage management features with ASM include

  • Striping
  • Mirroring
  • Asynchronous I/O
  • Direct I/O
  • SAME and Load Balancing
  • Is automatically installed as part of the base code set

ASM includes striping and mirroring to provide balanced and secure storage. The level of redundancy and the granularity of the striping can be controlled using templates. The new ASM functionality can be used in combination with existing raw and cooked file systems, along with OMF and manually managed files.

Direct I/O

By making use of Direct I/O, higher cache hit ratio can be achieved. Buffered I/O uses most important resources like CPU and memory. In case of buffered I/O Oracle blocks are cached both in the SGA and in the file system buffer cache.

Buffered I/O fills up the file system cache with Oracle Data, where as using the Direct I/O allows non-Oracle data to be cached in the file system much more efficiently.

Key Features and Benefits of ASM

The ASM functionality is controlled by an ASM instance.

The main components of ASM are disk groups, each of which comprise of several physical disks that are controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are know as ASM files.

The ASM divides a file into pieces and spreads them evenly across all the disks. The ASM uses an index technique to track the placement of each piece. Traditional striping techniques use mathematical functions to stripe complete logical volumes. The ASM includes mirroring protection without the need to purchase a third-party Logical Volume Manager. One unique advantage of ASM is that mirroring is applied on file basis, rather than on a volume basis. Therefore, the same disk group can contain a combination of files protected by mirroring, or not protected at all.

The ASM supports data files, log files, control files, archive logs, Recovery Manager (RMAN) backup sets, and other Oracle database file types. The ASM supports Real Application Clusters (RAC) and eliminates the need for a cluster Logical Volume Manager or a cluster file system.

Note:- ASM is shipped with the database and available as part of base code set and there is no need to go through a separate installation in the custom tree installation. It is available in both the Enterprise Edition and Standard Edition installations.

One of the flexible feature of ASM is it does not eliminate any existing database functionality which uses non ASM files. Existing database are able to operate as they always have been. New files may be created as ASM files, while existing ones are administered in the old way or can be migrated to ASM.

In ASM, at the top of the new hierarchy, you can find what are called ASM disk groups. Any single ASM file is contained in only one disk group. However, a disk group may contain files belonging to several databases, and a single database may use storage from multiple disk groups.

ASM files are always spread across all ASM disks in the disk group.

The ASM disks are partitioned in allocation units (AU) of on megabyte each. An AU is the smallest contiguous disk space that ASM allocates. The ASM does not allow physical blocks to split across AUs.

ASM General Architecture

To use ASM, you must start a special instance called an ASM instance before you start your database instance.

ASM instances manage the metadata needed to make ASM files available to ordinary database instances. Both ASM instances and database instances have access to a common set of disks call disk group. Database instances access contents of ASM files directly, communicating with an ASM instance only to get information about the layout of these files.

An ASM instance is like any other database instance except it contains two new background processes. First one coordinates rebalance activity for disk groups and it is called RBAL. The second one performs the actual rebalance activity for AU movements. At any given time there can be many of these, and they are called ARB0, ARB1, and so on. An ASM instance also has most of the same background processes as a ordinary database instance (SMON, PMON, LGWR, and so on.)

Each database instance using ASM has two new background processes called ASMB and RBAL. RABL performs global opens of the disks in the disk groups. At database instance startup, ASMB connects as a foreground process into the ASM instance. All communication between the database and ASM instances is performed via this bridge. This includes physical file changes such as data file creation and deletion. Over this connection, periodic messages are exchanged to update statistics and to verify that both instances are healthy

It is quite possible to cluster ASM instances and run them as RAC, using the existing Global Cache Services (GCS) infrastructure. There is one ASM instance per node on a cluster.

Storage in Oracle Real Application Clusters

Storage for RAC databases must be shared. In other words, datafiles must reside in an Automatic Storage Management (ASM) disk group, on a cluster file system, or on shared raw devices. This must include space for an undo tablespace for each instance if you are using automatic undo management. Additionally, for each instance you must create at least two redo log files that reside on shared storage. Oracle recommends, you can use one shared server parameter file (SPFILE) with instance-specific entries. Or you can use a local file system to store client-side parameter files (PFILEs).

If you do not use ASM, if your platform does not support a cluster file system, or if you do not want to use a cluster file system to store datafiles, then create additional raw devices as described in your platform-specific Oracle Real Application Clusters installation and configuration guide.

Automatic Storage Management in Real Application Clusters

ASM automatically optimizes storage to maximize performance by managing the storage configuration across the disks. ASM does this by evenly distributing the storage load across all of the available storage within your cluster database environment. ASM partitions your total disk space requirements into uniformly sized units across all disks in a disk group. ASM can also automatically mirror data to prevent data loss. Due to these added features, ASM significantly reduces administrative overhead.

As in single-instance Oracle databases, To use ASM in RAC, select ASM as your storage option when you create your database with the Database Configuration Assistant (DBCA).

Note:- using ASM in RAC does not require I/O tuning.

Automatic Storage Management Components in RAC

When you create your database, Oracle creates one ASM instance on each node in your RAC environment if one does not already exist. Each ASM instance has either an SPFILE or PFILE type parameter file.

The shared disk requirement is the only substantial difference between using ASM in a RAC database compared to using it in a single-instance Oracle database. ASM automatically re-balances the storage load after you add or delete a disk or disk group.

In a cluster, each ASM instance manages its node's metadata updates to the disk groups. In addition, each ASM instance coordinates disk group metadata with other nodes in the cluster. As in single-instance Oracle databases, you can use Enterprise Manager, DBCA, SQL*Plus, and the Server Control Utility (SRVCTL) to administer disk groups for ASM in RAC

Automatic Storage Management

ASM automatically optimizes storage to maximize performance by rebalancing the storage configuration across the disks that ASM manages. ASM spreads the storage load across all available storage within your cluster database environment for optimal performance. ASM partitions your total disk space into uniformly sized units across all disks in a disk group.

ASM functionality is controlled by an ASM instance.

The main components of ASM are disk groups, each of which comprise of several physical disks that are controlled as a single unit. The physical disks are known as ASM disks, while the files that reside on the disks are know as ASM files.

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