||Oracle Database Tips by Donald Burleson
Oracle10g Grid Computing
Chapter 5 -
Preparing Shared Storage
The performance of asynchronous I/O depends
much on if the kernelized asynchronous I/O or threaded asynchronous
I/O is used.
For kernelized asynchronous I/O, the kernel
allocates an asynchronous I/O request data structure and calls
an entry point in the device driver to set up the asynchronous
I/O request. The device driver then queues the physical I/O
operation and returns control to calling process. When the
physical I/O operation has completed, the hardware generates an
interrupt to a CPU. The CPU switches into interrupt service
context and calls the device driver's interrupt service routine
to update the asynchronous I/O request data structure and
possibly to signal the calling process with SIGIO.
The threaded implementation of asynchronous
I/O uses the kernel's lightweight process functionality to
simulate asynchronous I/O by performing multiple synchronous I/O
requests in distinct threads. This achieves I/O parallelism at
the expense of additional CPU usage associated with thread
creation and extra context switching overheads. If threaded
asynchronous I/O is used very intensively, these costs can add
as much as 5% to system CPU usage. For this reason, using
kernelized asynchronous I/O is a preferred method.
Kernelized Asynchronous I/O, popularly known as
KIO, is only available if the underlying file system uses Oracle
Disk Manager (ODM) API, Veritas Quick I/O, or a similar product that
routes the I/O via a pseudo device driver that can serve as the
locus for asynchronous I/O request completion. In addition, KIO is
available if you use the raw partitions. Many operating systems also
require special configuration of device files, device drivers and
kernel parameters to enable and tune kernelized asynchronous I/O. It
is definitely a complex configuration to achieve Asynchronous IO.
The above text is
an excerpt from:
Oracle 10g Grid & Real Application
Computing with RAC
by Mike Ault, Madhu Tumma
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