VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

Chapter 5, Using VERITAS Extension for Oracle Disk Manager
Prerelease 8 September 2005, 8:54am Understanding Oracle Disk Manager
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How Oracle Disk Manager Improves Database Performance
Oracle Disk Manager improves database I/O performance to VxFS file systems by:
◆ Supporting kernel asynchronous I/O
◆ Supporting direct I/O and avoiding double buffering
◆ Avoiding kernel write locks on database files
◆ Supporting many concurrent I/Os in one system call
◆ Avoiding duplicate opening of files per Oracle instance
◆ Allocating contiguous datafiles
Supporting Kernel Asynchronous I/O
Asynchronous I/O performs non-blocking system level reads and writes, allowing the system to
perform multiple I/O requests simultaneously. Kernel asynchronous I/O is better than library
asynchronous I/O because the I/O is queued to the disk device drivers in the kernel, minimizing
context switches to accomplish the work.
Supporting Direct I/O and Avoiding Double Buffering
I/O on files using read() and write() system calls typically results in data being copied twice: once
between the user and kernel space, and the other between kernel space and the disk. In contrast, I/O
on raw devices is copied directly between user space and disk, saving one level of copying. As with
I/O on raw devices, Oracle Disk Manager I/O avoids the extra copying. Oracle Disk Manager
bypasses the system cache and accesses the files with the same efficiency as raw devices. Avoiding
double buffering reduces the memory overhead on the system. Eliminating the copies from kernel
to user address space significantly reduces kernel mode processor utilization freeing more
processor cycles to execute the application code.
Avoiding Kernel Write Locks on Database Files
When database I/O is performed by way of the write() system call, each system call acquires and
releases a kernel write lock on the file. This lock prevents simultaneous write operations on the
same file. Because database systems usually implement their own locks for managing concurrent
access to files, write locks unnecessarily serialize I/O writes. Oracle Disk Manager bypasses file
system locking and lets the database server control data access.