Re: Some basic questions on udev usage

[Andrey Borzenkov <arvidjaar@mail.ru>]

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On Sunday 07 January 2007 18:18, Allexio Ju wrote:
>
> I am referring menu.lst file that is available on 2.6.19 vanila
> kernel. On the file, I thought I could modify 'root=/dev/sda2' to
> 'root=/dev/my-disk' from following entry in the file after I create
> new initrd image that has udev infra structure.
>     title Linux-2.6.19
>         kernel (hd0,1)/boot/Linux-2.6.19 root=/dev/sda2 vga=1
> resume=/dev/sda1 console=ttyS0,115200 console=tty0 nmi_watchdog=1
>
> Is this O.K?
>

If you ensure that initrd is able to create this name. Most persistent rules 
require non-trivial infrastructure (they call shell scripts that in turn call 
grep/sed/awk/...; they use *_id tools that are built against libc and 
libvolume_id and possibly require some more libraries). So you end up either 
maintaining two versions of rules (one for initrd one for booted system) or 
making initrd pretty big by putting large number of tools there. Both have 
obvious drawbacks although I'd say size of initrd is not that important 
now-a-days. More serious is simple observation that you do not have any 
automated way to find out what is required by udev rules.

It may be possible to automatically create specific rule for initrd that 
generates persistent name for a given root partition. I expect that when (if) 
major numbers become really dynamic, this is what will be done anyway.
akes sense?
>
> One more question.
> All .rules file under /etc/udev/rules.d start with number. If (really
> if) I want to have my own .rules file, what number should I consider
> as part of my file name?
> Are there any such guideline?

If you want to override default rules, add your own in the beginning. If you 
want to augment - add them in the end. Try to not override anything unless 
you know well what you are doing (as example - renaming disks away from sd* 
or hd* likely renders most tools that autodetect HDDs unusable). If you need 
to have specific name - *add* it as SYMLINK; never ever use NAME :)

- -andrey
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