||Oracle Database Tips by Donald Burleson
Oracle10g Grid Computing
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
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
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.
The above text is
an excerpt from:
Oracle 10g Grid & Real Application
Computing with RAC
by Mike Ault, Madhu Tumma
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