Prerequisites for Using ASM


This section looks at the prerequisites for using ASM and how it works in the database. Oracle Database 10g needs a separate smaller database instance (in a separate ORACLE_HOME) for ASM administration. This is created during the database setup and is known as an ASM instance. This instance manages the metadata needed to make ASM files available to regular database instances. ASM instances and database instances have access to a common set of disks called disk groups. Database instances communicate with the ASM instance to get information about the layout of ASM files and access its contents directly.

The ASM instance should be started first to create a database that will use ASM. An ASM instance can be shared by several database instances for using ASM files. In a RAC environment, only one ASM instance is needed per node regardless of the number of database instances on that node. The ASM instances communicate with each other on a peer-to-peer basis across the nodes.

When you install Oracle Database 10g on a system for the first time, the installation configures and starts a single-node version of the CSS daemon. The CSS daemon is needed for synchronization between the ASM instance and the database instances that depend on it for database file storage. It is configured and started regardless of your choice of ASM for storage. The CSS daemon must be running before you start any ASM instance or before a database instance starts, so it is configured to start automatically when the system boots.

For setting up multiple Oracle databases on a single server, you should run the CSS service and the Automatic Storage Management instance from the same ORACLE_HOME and use different homes for the database instances. With Oracle RAC installations, the CSS service is installed with Oracle Cluster Ready Services (CRS) in a separate ORACLE_HOME (called the CRS_HOME directory). For single-node installations, the CSS service is installed in the same ORACLE_HOME. So before you remove an ORACLE_HOME with an Oracle database, you must delete the CSS service configuration, or reconfigure the CSS service to run from another Oracle home directory.

ASM Architecture in a Nutshell

The basic component of Automatic Storage Management is the disk group. Oracle provides SQL statements to create and manage disk groups, their contents, and their metadata. A disk group is a group of (ASM) disks managed together as a unit. On UNIX/Linux systems, an ASM disk can be a block device, a network attached file (SAN), a logical unit number (LUN), or a LUN partition; on Windows-based systems, an ASM disk is always a disk partition. A disk group has its own file directory, disk directory, and other directories.

Inside a disk group, I/Os are balanced across all the disks. Dissimilar disks should be partitioned into separate disk groups for better performance. The redundancy characteristics (EXTERNAL, NORMAL, or HIGH REDUNDANCY) are set up when a disk group is defined. NORMAL REDUNDANCY is the default and prompts the disk group to tolerate the loss of a single failure without data loss. EXTERNAL REDUNDANCY means that ASM does not provide any redundancy for the disk group, while HIGH REDUNDANCY provides a greater degree of protection (three-way mirroring).

How Do We Group Disks on a System?

When you use more than one disk in a disk group, the disks should have similar size and performance characteristics. Group the disks according to their size and usage type (database files, control files, flash recovery areas, and so on). For storage array disks, do not divide the physical volumes into logical volumes. To the ASM instance, any type of disk division will hide the physical disk boundaries and hinder operational performance.


When you start the ASM instance, it will automatically discover all available ASM disks. ASM discovers disks in the appropriate paths that are listed in an initialization parameter or in an operating systemdependent default path (if the initialization parameter is NULL).

You need to define the failure groups in a disk group whenever you create or alter the disk group.


ASM uses failure groups to identify the disks (for normal and high-redundancy disk groups) that share a common potential failure mechanism, such as a set of SCSI disks using the same controller. Failure groups determine which ASM disks are to be used for storing redundant copies of data.

Files written on ASM disks are known as ASM files. A data file can be stored as an ASM file, a file system file, or as a raw device. ASM files can be created for redo log files, temporary files, RMAN files, parameter files, and data pump dump files.

How Do We Name an ASM File?

The ASM filename format depends on the context of file usage, such as referencing an existing file, creating a single file, or creating multiple files. You can use fully qualified filenames with a disk-group name, a database name, a file type, a type-specific tag, a file number, and an incarnation number. The fully qualified name is generated for every ASM file on its creation. Numeric names can be derived from fully qualified names. The DBA can also specify user-friendly alias names for existing as well as new ASM files. Incomplete filenames with disk-group names are used only for ASM file-creation operations. Refer to Oracle Database Administrator's Guide 10g for more details.


Each ASM file will belong to only a single disk group. A disk group may contain files belonging to several databases, and a single database may form multiple disk groups. ASM files are always spread across all the disks in the disk group. When an ASM file is created, certain file attributes such as a protection policy (mirroring or none) or a striping policy are set. These files are visible to RMAN and other Oracle-supplied tools. In Oracle Database 10g Release 2, new features have been added to make them visible to the operating system and its file-movement utilities.

Oracle uses templates to simplify database file creation by mapping complex file attribute specifications about ASM files (in ASM disk groups) to a single name. A disk-group template is a collection of attributes that are applied to all files created within the disk group. Oracle provides a set of initial default templates for use by ASM. The v$asm_template view has a list of all templates identifiable by the ASM instance. You can add new templates to a disk group, modify existing ones, or even drop existing ones using the ALTER DISKGROUP statement.



    Oracle Database 10g Insider Solutions
    SUSE LINUX Enterprise Server 9 Administrators Handbook
    ISBN: 672327910
    EAN: 2147483647
    Year: 2006
    Pages: 214

    flylib.com © 2008-2017.
    If you may any questions please contact us: flylib@qtcs.net