16
Maxim
ize p
erformanc
e
available in the file
Stripe.txt
on the CD which has
been included with your ATTO ExpressPCI
adapters.
Increase transfer size
If you are writing to an application that uses a lot
of sequential disk I/O to a contiguous area on
disk, you should use as large a transfer size as
possible to reduce overhead on the system, on the
SCSI bus and within disk drives.
Analyze your I/Os
For large sequential data transfers, use Direct I/O
by selecting
FILE_FLAG_WRITE_THROUGH
and
FILE_FLAG_NO_BUFFERING
flags with
your
CreateFile
call to avoid the overhead of
copying data from one area of memory to another,
to reduce the number of SCSI commands which
must be executed and to leave system pages
available for other data.
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 will get
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,
you will see marked performance
degradation.
❖
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, you will see
performance improvements in the 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.
