The DMA Issue in RedHat Linux 7.x
Shanker Balan and Atul Chitnis
RedHat ships a highly tuned version of the Linux kernel which enables
DMA (Direct Memory Access) or even Ultra DMA on selected IDE controller
chipsets.
DMA based transfers greatly reduce install time by
providing faster file transfer speeds. DMA also reduces CPU usage to a
large extent for disk activity.
Because of the above mentioned benefits, DMA is a highly desirable feature
and in most cases, the DMA enabled kernel works like a charm.
(For more on the benefits of using DMA transfers see
http://www.linuxdoc.org/HOWTO/mini/Ultra-DMA.html)
However, enabling DMA has its downside. It requires that all the components of your
system are "healthy". If there is a hardware related problem in any one of
the related system components, DMA will fail to function properly.
Slow and unreliable installs, CRC errors, file corruptions and even
random system lockups are all symptoms of DMA related problems.
The most common issues seen are
- the installer dies when booting, after issuing the message
"running: /sbin/loader"
- installation is extremely slow and often dies at a particular point, but
never the same for different users/machines
- installation suddenly stops complaining about a missing/bad file
- the CDROM is not accessible after installation
Q. Ok, so how can I fix the installation problem?
A. To fix the problem, boot off the CD, then, at the boot prompt type
linux ide=nodma
This disables DMA access while installing.
Q. Whew, I disabled DMA with "ide=nodma" and the install completed! But
now I seem to be having problems while running normally? Is this also
DMA related?
A. Most probably yes as the "ide=nodma" option does not disable DMA
permanently - it only disables DMA for the current session.
Q. So how do I disable DMA permanently?
A. Depending on how the CDROM drive has been physically connected to your
system, add one of these lines to /etc/rc.d/rc.sysinit:
/sbin/hdparm -d0 /dev/hdb # Add this if your CDROM is on primary slave
/sbin/hdparm -d0 /dev/hdc # Add this is your CDROM is on secondary master
/sbin/hdparm -d0 /dev/hdd # Add this if your CDROM is on secondary slave
If you are unsure about the CDROM device id, the "dmesg" output will
have it:
~# dmesg|grep CD
hdc: ATAPI 48X CDROM, ATAPI CD/DVD-ROM drive
hdc: ATAPI 48X CD-ROM drive, 128kB Cache,
UDMA(33) Uniform CD-ROM driver Revision: 3.12
In this case, CDROM drive is /dev/hdc.
Q.I disabled DMA for the CDROM drive. I still have problems while
accessing files. Everything is too slow and the hardisk light remains on
for a looong time
A. Looks like your harddisk also cannot handle DMA.
To disable DMA for a hard drive, add this line to /etc/rc.d/rc.sysinit.
/sbin/hdparm -d0 /dev/hda
This should cure most DMA related problems.
Q. How can I fix DMA related problems so that I can take advantage of
its benefits?
A. Some of the more common reasons for DMA problems are:
Q. Where can I find out more about Linux and (U)DMA?
A. See the The Linux Ultra-DMA Mini-Howto at
http://www.linuxdoc.org/HOWTO/mini/Ultra-DMA.html
Q. But other distributions like SuSE and Debian install fine on the same
machine. Even RedHat Linux 6.2 which PCQuest distributed work fine. Why
so?
A. Other Linux distribution like Debian and SuSE do not enable DMA by
default on the installer (or on their default kernels) just like the
earlier releases of RedHat Linux which is probably why you never had a
similar problem before.
On these systems, you can enable DMA manually using the "hdparm" command.
See the PCQuest article Fine tuning your Linux machine
http://www.pcquest.com/archive/articledetail.asp?arid=11733
Update:
In response to a posting about this article on Linux Weekly News,
RedHat has added the following information:
From: "Michael K. Johnson" <johnsonm@redhat.com>
To: editor@lwn.net
Subject: Comment on exocore article
Date: Mon, 09 Jul 2001 12:18:09 -0400
You pointed to an article regarding IDE DMA being enabled in
Red Hat Linux 7.1 at http://www.exocore.com/linux/rhl71dma/
There is one critical piece of information to add to that article.
If you need to use the ide=nodma flag to install, please, PLEASE
send us the model information for your CDROM drive so that we can
add it to the "blacklist" of CDROM drives that are incompatible with
DMA. This will make your life easier installing new versions of
Red Hat Linux, and will help everyone else with that model as well.
In order to provide us with that information, submit a bug report via
http://bugzilla.redhat.com/bugzilla
Assuming your CDROM drive is "hdc", include the exact output of
cat /proc/ide/hdc/model
Thanks!
I'd send this email to exocore as well except that they insist on
web forms rather than providing an email address for feedback.
michaelkjohnson
"He that composes himself is wiser than he that composes a book."
Linux Application Development -- Ben Franklin
http://people.redhat.com/johnsonm/lad/
It would have been nice if RedHat would have provided an email address
to send such information to, rather than forcing people to register and
fight their way through bugzilla
to deliver such information.
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