Re: [dm-crypt] renaming of device

[Arno Wagner <arno@wagner.name>]

On Fri, Jan 15, 2010 at 04:30:11PM -0800, Ross Boylan wrote:
> On Fri, 2010-01-15 at 04:58 +0100, Arno Wagner wrote:
> > 
> > Good idea, but no. Must be something in the udev configuration
> > after all. Not that I found anything. Or anything in the logs.
> > Hmm. Oh, well. It is not that important. But I really hate this
> > badly documented obscure "automagical" stuff. As soon as anything
> > breaks, it really sucks. 
> > 
> > On the side of where cryptsetup finds the device for a specific
> > major and minor number, it is indeed a simple recursive directory
> > traversal. Unfortunately it is in libdevmapper.c, i.e. in the 
> > system lib of that name. That means it cannot easily be changed.
> I have a comment and a question.
> 
> The comment is that I think udev on Debian has some private place that
> it stashes away mappings so that network cards (for sure) and disks (I
> think) have stable names.  I think it's a Debian-specific feature,
> though other distros may do something similar.

May well be that way. However I am pretty sure I do not have stable
disk names. I did have stable network card names, but disabled
that feature, since I swap network cards from time to time
and need to manually adjust anyways.
 
> The question concerns the use of major:minor to identify devices.  I
> thought the same physical device can have different major:minor on
> different boots.*  Is that so, and if so, can it cause problems for
> dm-crypt, such as causing it to pick the wrong underlying volume?

dm-crypt does not do any aoutomation, but if a boot script
were to give the same device to dm-crypt when the mapping has 
changed, that will cause problems. However the mapping can also
change when changing the partitioning, with much the same effects.


> I'm pretty ignorant in this area so a) take what I say with a grain of
> salt and b) I'd appreciate any wisdom.
> 
> Thanks.
> 
> Ross Boylan
> 
> *I think the Debian stuff stabilizes the relation between physical
> devices and names like /dev/sdd, but not necessarily physical devices
> and major:minor.

It does. Each possible /dev/sd<x> has a defined, fixed 
major/minor, see Documentation/devices.txt in a Linux kernel 
source tree.

Arno 

-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
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