Veritas File System 5.1 SP1 Administrator"s Guide (5900-1499, April 2011)

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
About kernel asynchronous I/O support
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.
About direct I/O support 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.
About 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.
107Using Veritas Extension for Oracle Disk Manager
About Oracle Disk Manager