November 23, 1998
How to use Windows' DMA check box to speed up your drives
My past two columns have described problems that a few Windows 95/Windows
98 and Windows NT users have been having with hard drives that use Direct
Memory Access (DMA) or the faster Ultra DMA (UDMA) standard. If you're
experiencing these problems, those two columns provide work-arounds.
I hope my columns, however, haven't scared you away from using DMA if
your system is capable of it. This technique can improve your hard disk
performance.
To see whether your system is capable of hard disk DMA/UDMA, run the
System Control Panel and click the Device Manager tab. Double-click the
Disk Drives icon, then double-click a disk drive and click the Settings
tab. If you see a DMA check box, drivers are present that are DMA-capable.
If not, you do not have such drivers.
Windows 98 and Windows 95B automatically install DMA-capable drivers
if they detect suitable hardware. It is also possible to install such drivers
under Windows 95, Win 95A, and NT (using Service Pack 3).
If you see a DMA check box and it is not checked, you may be able to
improve your hard disk performance by turning on this capability. Before
you do this, however, you should do two other things.
-
Make an emergency boot diskette and verify that it works to start up your
system, in case enabling DMA causes some problem. (If this happens, start
Windows in Safe Mode, turn the DMA check box off, then reboot.)
-
Get a utility that reports on hard drive performance, including DMA performance,
so you can see the difference you get. A good utility is HD Tach, a program
available at tcdlabs.simplenet.com/hdtach.htm.
The trial version of HD Tach tests your drives' read performance, and the
registered version, priced at $49.95, also tests write performance.
I used HD Tach to test two hard drives with and without DMA enabled. One
drive improved its read burst speed from 8.4MBps to 11.9MBps. The other
improved from 8.3MBps to 11.2MBps. More important, the CPU utilization
dropped from 83 percent to 29 percent on one drive and from 61 percent
to 20 percent on the other. Lowering the CPU utilization of a drive allows
a computer to process more data or serve more users at the same time as
disk files are being transferred.
I asked Microsoft for a definitive answer on whether or not Windows
98 automatically enables DMA. It turns out that Windows' DMA drivers, when
installed for the first time, do try to enable DMA but may disable it if
your system fails certain tests. Specifically, the drivers (1) query the
motherboard chip set, (2) query the drive itself, and then (3) test a short
pattern of disk reads and writes to see if they are reliable at DMA speeds.
Microsoft company representative David Alles put it this way: "On a
machine that is upgraded to Windows 98, we retain the DMA settings (or
lack thereof) of the previous state. If it was Windows 95 Gold, DMA will
be off. But if a user had an OSR2 [Windows 95B] machine and had turned
on DMA, it will remain on in Windows 98."
"When users check the DMA box in Device Manager, sometimes it appears
unchecked after the system reboots. In such a case, we have determined
that at least one of the three criteria mentioned above has not been met,
so the system is not suitable for DMA," Alles continues.
To test whether a drive supports DMA, run \Tools\Reskit\Help\Rk98book.chm
on the Windows 98 CD-ROM. Search on "PIO mode 4" and read the resulting
topic.
Brian Livingston's latest book is Windows 98 Secrets (IDG Books).
Send tips to brian_livingston@infoworld.com.
He regrets that he cannot answer individual questions.
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© 1998 InfoWorld Media Group Inc.
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