Re: [linux-lvm] mount's (&udev's) dirty little naming problem

From: Linda A. Walsh
Date: Tue Jul 19 2011 - 16:51:56 EST

Luca Berra wrote:
On Tue, Jul 19, 2011 at 12:57:21AM -0700, Linda A. Walsh wrote:
(sorry for the length of this, in advance, but I've tried to give
this some though...(maybe not enough/maybe too much!) ;-) )

Luca Berra wrote:
> On Mon, Jul 11, 2011 at 03:16:31AM +0100, Alasdair G Kergon wrote:
>> The canonical name to use is /dev/<vgname>/<lvname>. ...
>> [~Other forms - dm-N and /dev/mapper/X-Y are internal and ...
>> there'll probably be some further changes to workaround other
>> arbitrary udev restrictions...~]
>> Different versions of df and mount haven't always retained this
>> preferred form for their output.
> note that on a freshly installed rhel 6.1 i have this in fstab
> /dev/mapper/vg_candela--lv_root / ext4 ....


Why are there two dashes between the VG&LG?
woops, typo, sorry for that it is :
/dev/mapper/vg_candela-lv_root / ext4 defaults 1 1

That's consistent with current behavior -- the problem is
in the current name mangling (i,e. mounts by "/dev/VG/LV" get
xlated into "/dev/mapper/<mangled-VG>-<mangled-LV>", where
at least one manglement is xlating "-" into "--". But I've already been
told that future manglements may be added due to limitations in udev.

For non LV devs, it would be like having '/dev/sdc2' xlated
into '(hd2,<rand>)' (where <rand> = some not always consistent integer)
which, of course would be silly! ( ;-) ), or xlating
LABEL="MyDisk-Label" => '/dev'disk/by-label/MYDISK--LABEL'
(with 'My-Label' being 'canonicalized', for some definition of thereof,
into some other value), vs. now, it's given the real devname
(/dev/sdc<x>) (which is FINE with me...; it's the real HW name that
the kernel uses, whereas 'label=xxx was a syntax I was encouraged
to use as part of the suse boot system, as 'everything works with labels'
(well, lets not speak about lilo, or booting from an HD w/o a ram disk,
as we'll just drop support for those options...*cough*)....

As noted, the current behavior isn't giving/using either
the 'real devname' /dev/dm-[x], NOR the user name, but a mangled name making it the worst of all worlds, as there is no easy way to find
out the LV and VG of the device mounted on a given mount point.

To get around this == and tell me if I'm wrong, I have to use
lvm, to output a list of all VG,LV groups.

Then using a [perl] script (I suppose BASH's Associative arrays might
work too, but familiarity points toward perl at this point), I can
build up an Assoc array to map devno's -> VG,LV, then use the devno
of the mountpoint as an index into that.

That level of complexity is so completely LAME just to find out
the name I used to mount a FS on some mount point!....

If there is an easier way, please, hit me with a clue stick!


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