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"Rudi Ahlers" 11-16-2008 11:05 PM

is udev necessary?
Hi all

I recently setup a CentOS 5.2 server, running XEN (using HyperVM), and
then moved the hard drive from my test box to my Intel server.The
problem I now have, is that it doesn't bootup properly. Shortly after
I see the udev service started, the machine reboots. This keeps on
going the whole time.

I have managed to kill udev on start-up (with CTRL + C), and then it boots up.
So, do I need udev? And what is it's purpose?


Kind Regards
Rudi Ahlers
CentOS mailing list

Berend Dekens 11-16-2008 11:32 PM

is udev necessary?
Rudi Ahlers schreef:

I have managed to kill udev on start-up (with CTRL + C), and then it boots up.
So, do I need udev? And what is it's purpose?
Udev is a device probing layer. In the old days we had a /dev system
prepped for standard use which would be complemented with other bootup
scripts to make nodes for all hardware in your system.

Udev is the successor of this system (the one line history version
anyway) and builds up the /dev folder with all your devices. In theory
this is great but most systems (and I'm fairly sure CentOS as well)
still have a number of base nodes in /dev before udev is fully started.
This helps the system boot and in case of emergency (udev crashing or a
broken probing like you have) this would allow the system to boot and
find its primary devices (if you are lucky this might include all
neccesary devices, in my case for example, when udev won't start I only
have one of 2 SATA controllers online).

So in short, you might be able to turn off udev but adding new hardware,
plugging in usb devices or similar or starting some non-standard
hardware won't work any more. Perhaps there are more serious issues
(like soft-raids ignoring the raid and just using one drive).

You might be able to see in the kernel console (ctrl+f10) what happens
just before the system reboots - if it is a module which fails (most
likely) you could blacklist it. That would solve the reboots. If the
module is in fact critical for some piece of hardware you might be able
to tweak it instead of disabling udev altogether.

Do the system logs contain any clues what is going on or does the system
kills itself before logging to harddisc comes on?

Berend Dekens
CentOS mailing list

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