Product guide

Maximizing performance in Windows

If your application requires a small number of

I/Os and the transfers are rather small, however,

you may get better performance by letting the

system cache your data in the system pages.

You might want to use over-lapped I/O using the

FILE_FLAG_OVERLAPPED

option with

the

CreateFile

call. Overlapped I/O allows the

application to send many commands to the device

at once.

The ATTO Disk Benchmark program, included

with the ExpressPCI Utilities on the CD shipped

with your ExpressPCI adapter, shows the effect of

using the above I/O modes.

• If you turn off Direct I/O and set the file size to a

value significantly less than the amount of

memory in your computer, you may observe

some artificially high transfer rates because very

little I/O is actually being performed by the SCSI

device: all the activity is involved in transferring

data between the application and the system

pages.

• As you increase the file size to a value more than

the amount of memory in your computer,

performance degrades significantly.

• If you turn on Direct I/O, you can see the effect

of removing the system pages from the

overhead picture.

• If you use overlapped I/O, performance

improves in low to medium transfer sizes.

• However, depending on the amount of memory

in your computer, you may not be able to use

queue depths greater than 4 or 5.