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Donald K. Burleson

Oracle RAC Tips

Oracle Disk Manager

Oracle Disk Manager (ODM) is Oracle’s innovative I/O and file management infrastructure. Oracle Disk Manager is a very significant development in the way Oracle manages and interfaces with the 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 Oracle.

Many of the leading file system and volume manager providers have incorporated the ODM interface in their products. Well known examples include Veritas Database Edition (Advanced Cluster for Oracle RAC) and Polyserve MxS Oracle RAC Option. Several ODM semantics have been included in the DAFS v1.0 protocol specifications offered by Network Appliance Filers.

Let us look at some of the ODM features: [Musi]

  • 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. Rather, 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.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.





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