VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

Understanding Oracle Disk Manager Prerelease 8 September 2005, 8:54am
126 VERITAS Storage Foundation for Oracle Administrator’s Guide
Supporting Many Concurrent I/Os in One System Call
When performing asynchronous I/O, an Oracle process may try to issue additional I/O requests
while collecting completed I/Os, or it may try to wait for particular I/O requests synchronously, as
it can do no other work until the I/O is completed. The Oracle process may also try to issue requests
to different files. All this activity can be accomplished with one system call when Oracle uses the
Oracle Disk Manager I/O interface. This interface reduces the number of system calls performed to
accomplish the same work, reducing the number of user space/kernel space context switches.
Note With Oracle9i Release 2 (9.2) or later, you can use the FILESYSTEMIO_OPTIONS
initialization parameter to enable or disable asynchronous I/O, direct I/O, or Concurrent I/O
on file system files. This parameter is applicable to JFS and JFS2 files only. This parameter
is not applicable to VxFS files, ODM files, or QIO files. For further information, please refer
to Oracle Disk Manager documentation provided by Oracle.
Avoiding Duplicate File Opens
Oracle Disk Manager allows files to be opened once, providing a “file identifier.” This is called
“identifying” the files. The same file identifiers can be used by any other processes in the Oracle
instance. The file status is maintained by the Oracle Disk Manager driver in the kernel. The
reduction in file open calls reduces processing overhead at process initialization and termination,
and it reduces the number of file status structures required in the kernel.
Allocating Contiguous Datafiles
Oracle Disk Manager can improve performance for queries, such as sort and parallel queries, that
use temporary tablespaces. Without Oracle Disk Manager, Oracle does not initialize the datafiles
for the temporary tablespaces. Therefore, the datafiles become sparse files and are generally
fragmented. Sparse or fragmented files lead to poor query performance. When using Oracle Disk
Manager, the datafiles are initialized for the temporary tablespaces and are allocated in a
contiguous fashion, so that they are not sparse.