[CentOS] is udev necessary?

Berend Dekens berend at cyberwizzard.nl
Mon Nov 17 00:32:39 UTC 2008

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

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