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Changes in device-mapper and LVM2 that can affect grub's functionality

From: Peter Rajnoha
Subject: Changes in device-mapper and LVM2 that can affect grub's functionality
Date: Wed, 09 Sep 2009 12:01:33 +0200
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(I'm speaking for the LVM/DM team)

we have integrated official udev support for device-mapper/LVM
devices lately which is now part of upstream LVM release (however
it's still turned off in default configuration). This includes
our own udev rules responsible for creating /dev nodes and
associated symlinks.

After discussing this with the udev team, we decided to create
DM nodes directly in /dev (with the kernel name of that particular
DM device, which is "dm-X", X being a number for now). The /dev/mapper
contains symlinks to these nodes then. This seems to be a standard
solution from udev point of view which is considered to be correct.
We would like to comply with this standard.

As for the LVM subsystem, the symlinks stay like in the old non-udev
approach -- /dev/VG_NAME subdir containing symlinks with LV names that
point to appropriate DM devices. The change here is only in their
target -- they point to /dev/dm-X devices now, not /dev/mapper ones.

However, we have to consider that not all configurations will have
this udev support turned on, therefore we still keep the old code
responsible for direct creation of the nodes/symlinks (this feature
can be turned on/off by particular configuration option).

Also, if we detect that udev daemon is not running or node/symlink
creation has failed for any reason, we fallback to the old way
of node/symlink creation.

To sum it up briefly, this means we have two layouts that should
be supported:

1. old layout -- nodes /dev/mapper/DM_NAME,
                 symlinks /dev/VG_NAME/LV_NAME

2. new (udev) layout -- nodes /dev/dm-X,
                        symlinks /dev/mapper/DM_NAME
                        symlinks /dev/VG_NAME/LV_NAME

Such layout breaks grub-probe because of the symlinks used afaik
(particularly "find_root_device" function that is used in this

My question is if supporting this would be a problem from grub's
point of view and if appropriate corrections could be made.

Also, I've spotted that you use some hardcodings in your code,
particularly when filtering out /dev/dm-[0-9] devices
(e.g. in "find_root_device" fn). The thing is that this number
part of the DM name could be changed in the future, so such
assumptions should not be made as well.

I'd like to open a discussion here and any comments are welcome
so finally we could end up with a working solution.



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