Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

Oracle Disk Manager

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() to support 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.

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