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General tuning techniques 16

General tuning techniques

Using direct I/O, unbuffered, or zero copy 10

Traditional I/O paths include the page cache, a DRAM cache of data stored on the disk. The IO Accelerator

is fast enough that this and other traditional optimizations, such as I/O merging and reordering, are actually

detrimental to performance. I/O merging and reordering are eliminated naturally by the IO Accelerator, but

the page cache must be bypassed at the application level.

Direct I/O bypasses the page cache. This allows the same memory regions written by the application to be

DMA-transferred or copied directly to the IO Accelerator rather than having to be copied into kernel-owned

memory first, thus eliminating a copy.

Bypassing the page cache provides the following benefits:

• Less complex write path

• Lower overall CPU utilization

• Less memory bandwidth usage

In most cases, direct I/O is beneficial for IO Accelerator-based systems, but you should benchmark your

application to be sure of this. Some applications that do not use AIO, threads, or multiple processes to create

multiple outstanding requests might benefit from the page cache instead.

Many I/O-intensive applications have tunable parameters that control how they interact with the low-level IO

subsystem, including turning on direct I/O.

For other applications, it is necessary for the application provider to enable direct I/O or to modify the source

to enable direct I/O, and then recompile.

For a more in-depth look at how to write C code using Direct IO, see "Programming using Direct IO."

dd support

More recent versions of dd support the oflag=direct and iflag=direc}}t parameters. These enable

direct I/O for either the file being written to or the file being read from, respectively. Use the

{{oflag=direct parameter when writing to an IO Accelerator and the iflag=direct parameter when

reading from an IO Accelerator.

ioZone Benchmark

ioZone supports the -I option to enable Direct I/O.

fio Benchmark

Fio uses the direct=1 setting in the job file, or --direct=1 on the command line to enable Direct I/O.

Multiple outstanding IOs

The IO Accelerator is more like a storage controller than a single disk. Like other storage controllers, it

performs best when multiple requests are outstanding. Unlike other storage solutions that rely on legacy

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