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Oracle Disk Manager (ODM)

Oracle RAC Cluster Tips by Burleson Consulting

Oracle Disk Manager (ODM)

Oracle Disk Manager (ODM) is a disk management interface defined by Oracle to enhance file management and disk I/O performance. Implementation of an ODM interface in a file system or a logical volume manager (LVM) provides many benefits including simplified file administration, improved file integrity, and reduced system overhead. It is completely transparent from an application/DBA perspective. It is an improvement over standard UNIX I/O.

ODM eliminates file descriptors, which simplifies the operating system kernel configuration and saves operating system resources. Other ODM capabilities include special locking modes that prevent errors and the ability to pass usage hints, such as Oracle file type information and I/O priorities for third party application integration. It manages all I/O type files on both system files and raw partitions with one system call, which is not possible with native operating system calls.

Some of the visible advantages include:

* Files being created/initialized are not visible until they are committed.

* Files must be identified or opened with a common key and cannot be removed if they are in use.

* Works with regular files and raw volumes.

Oracle Database 10g automatically takes advantage of the ODM interface when the underlying file system or logical volume manager becomes ODM enabled. A file is considered an ODM file if it is in a file system that supports the ODM interface. ODM is also compatible with the new Oracle Real Application Clusters.

Many of the file system/volume manager vendors have incorporated the ODM interface. Well known examples include, Veritas Database Edition (Advanced Cluster for Oracle Database 10g RAC) and Polyserve Matrix Server for Oracle 10g RAC. Several ODM semantics have been included in the DAFS v1.0 Protocol Specifications offered by Network Appliances.


Oracle Disk Manager (ODM) was Oracle 9i's innovative I/O and file management infrastructure. Oracle Disk Manager is a very significant development in the way Oracle manages and interfaces with I/O activity. The ODM interface is a set of API calls that Oracle co-developed with Veritas for incorporating underlying volume managers and file systems. Implementation of the ODM interface in a file system or a logical volume manager provides many benefits, including simplified file administration, improved file integrity, and reduced system overhead.

ODM is completely transparent from an application/DBA perspective. It is an improvement over standard Unix I/O. The ODM interface allows the Oracle kernel to allocate/release disk space, manage tablespaces, and read/write disk blocks directly. Oracle encourages customers to use file system or volume managers that are ODM compliant.

Oracle automatically takes advantage of the ODM interface when the underlying file system or logical volume manager becomes ODM-enabled. A file is considered an ODM file if it is in a file system that supports an ODM interface. ODM is also compatible with Oracle9i and 10g. Many of the leading file system and volume manager providers have incorporated the ODM interface into their products. Well known examples include Veritas Database Edition (Advanced Cluster for Oracle) and Polyserve Matrix Server. Several ODM semantics have been included in the DAFS v1.0 protocol specifications offered by Network Appliance Filers.

The following is an examination of some of the ODM features:

* Without ODM, Oracle must resort to many different sets of calls to manage the wide variety of I/O types. For example, Oracle uses calls such as pwrite(), pread(), async_write(), readv(), read(), write(), lio_listio(), and kaio(). With ODM, Oracle needs only the single call odm_io(). odm_io() supports all Oracle file I/O types on ALL files (Raw or VxFS).

* Normally, asynchronous DBWR page flushing requires two calls, one to issue the I/O and another to poll for completed I/O. With ODM, gathered writes (DBWR) and LGWR asynchronous writes occur with a single call to odm_io() without regard for file type (VxFS or RAW) or number of target files. Checking for completed I/O requests is conducted while issuing new requests.

* ODM includes features that enable more effective Oracle file creation. Without ODM, failed attempts to add files to a database can result in an unused file that must be cleaned up from outside Oracle. With ODM, files are no longer created with traditional open() or create() calls.

* Files are created with odm_create() and then initialized or filled. If the file creation is a success, it then calls odm_commit(). If there is failure, Oracle calls odm_abort(). The file will be completely cleaned up from within ODM.

* With ODM, Oracle no longer uses file descriptors. Instead, ODM identifiers are used. ODM identifiers are shareable from process to process within the node. Oracle caches ODM identifiers in the SGA at instance startup. ODM identifier usage reduces kernel overhead.

Veritas and Oracle studies indicate that the ODM files perform equal or better than the raw partitions. ODM yields roughly 8% reduction in kernel mode CPU utilization on certain platforms under certain workloads.

ASM (Automated Storage Management) and ODM

ASM divides files into 1 MB extents and spreads the extents for each file evenly across all of the disks in a disk group. ASM does not use a mathematical function to track the placement of each extent; it uses pointers to record extent location. This allows ASM to move individual extents of a file when the disk group configuration changes without having to move all extents to adhere to a formula based on the number of disks.

As said above, for normal datafiles, ASM uses 1 megabyte extents. For log files, that require low latency, ASM provides fine-grained (128k) striping to allow larger I/Os to be split and processed in parallel by multiple disks. The DBA decides at creation time whether or not to use fine-grained striping. File-type specific templates in the disk group determine the default behavior.

 

Configuration of Automated Storage Management

One of the most significant new features in Oracle 10g release is the introduction of Automated Storage Management (ASM). In order to use the ASM in a shared mode for the use of RAC database, an ASM instance needs to be created for each node of the RAC database involving the same disk devices as input. Each ASM instance has either an SPFILE or PFILE type parameter file.  To use ASM in the RAC environment, select ASM as the storage option when creating the database with the Database Configuration Assistant (DBCA).

 
   
Oracle Grid and Real Application Clusters

See working examples of Oracle Grid and RAC in the book Oracle Grid and Real Application Clusters.

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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