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

[Linda A. Walsh]

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
>
> L. 
------
  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!

-l

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