missing device nodes for vg, but vg appears in scan

[RoundsToZero]

I am using lvm2 2.01.09, gentoo-sources 2.6.15-r1, and udev 079. I didn't get
a response to this as a reply to a previous thread which seems to have fallen
off the face of the earth, and I figured this deserved its own thread.

I am trying to set up an LVM2 VG using a loop device. My problem is that
device nodes are never created for my VG. However, my VG shows up in scans
(vgscan) and displays (vgdisplay). Even vgscan --mknodes does not make the
nodes. Because I never get a node for my VG, I can't figure out how I am
supposed to create LV's in it.

I have the modules loop, dm-mod and dm-snapshot loaded. Therefore, I do
have /dev/mapper/control. Here is what I have done so far:

• Create file and set up to use with loop device
  # dd if=/dev/zero of=/local/snapshot bs=1M count=512
  # losetup /dev/loop0 /local/snapshot
  
• Set up the PV
  # pvcreate /dev/loop0
  
• Create the VG
  # vgcreate snapshot /dev/loop0
  

At this point commands like this fail:
# lvcreate -L512 -s -n backup snapshot
Path required for Logical Volume "snapshot"
The origin name should include the volume group.
# lvcreate -L512 -s -n backup /dev/snapshot
"/dev/snapshot": Invalid path for Logical Volume
The origin name should include the volume group.

I have read in the docs that "vgscan" is run automatically now and that I
should expect to see my VG in /dev. This does not happen so I do the
following:

• Try to activate the VG
  # vgchange -a y snapshot
  0 logical volume(s) in volume group "snapshot" now active
  
• Try running vgscan manually
  # vgscan
  Reading all physical volumes. This may take a while...
  Found volume group "snapshot" using metadata type lvm2
  # vgscan --mknodes
  Reading all physical volumes. This may take a while...
  Found volume group "snapshot" using metadata type lvm2
  
• No results in /dev, try vgdisplay
  # vgdisplay
  

  Code:

  --- Volume group ---   VG Name               snapshot   System ID   Format                lvm2   Metadata Areas        1   Metadata Sequence No  1   VG Access             read/write   VG Status             resizable   MAX LV                0   Cur LV                0   Open LV               0   Max PV                0   Cur PV                1   Act PV                1   VG Size               508.00 MB   PE Size               4.00 MB   Total PE              127   Alloc PE / Size       0 / 0   Free  PE / Size       127 / 508.00 MB   VG UUID               RG5GM3-WrEZ-PmCM-Fl0A-wjAy-Udqo-xLASVj

  
  

It looks like my VG is there, but it still isn't showing up in /dev. I read
something about how if udev isn't configured properly, you will get devices
like /dev/dm-# where # is 0-9, instead of named VG nodes. I don't have those
either, but I follow the instructions which are to uncomment a line in my
udev rules:

KERNEL=="dm-[0-9]*", PROGRAM="/sbin/devmap_name %M %m", NAME="%k",
SYMLINK+="%c"

I learn that devmap_name is included with multipath-tools, so I install those,
version 0.4.6. I reload my udev rules and repeat the steps above, but there
is no change. Then I wonder if this has anything to do with the multipath
target in the kernel configuration for the device mapper. I enable it,
reboot for good measure, and load the multipath module. I repeat the steps
above, but there is still no change.

At this point I wonder if udev has some problem outside the scope of my
imagination, so I try to see if I can use vgscan to create nodes outside
of /dev by editing lvm.conf. When I run vgscan --mknodes, however, it still
only creates mapper/control. I guess I shouldn't have really expected that
to work, but it's still weird.

[zeroclip]

Exactly the same problem here. I've tried everything you have I end up with the same result!

Please help someone!!

[zeroclip]

Huh. Now it works!

All i did was update to udev-81-r1, uncomment the dm-[0-9] rule in 50-udev.rules and then a reboot. Now it works! Strange u

[RoundsToZero]

Do you still have multipath-tools installed? And the multipath kernel module?

[zeroclip]

yep same kernel 2.6.15-gentoo-r1 and multipath-tools installed.

[RoundsToZero]

It's still not working for me. Am I supposed to have a /proc/lvm? I don't, but when I did an strace of vgscan it looked like it was trying to access it a lot and was unable to.

EDIT: Also, do you need the multipathd service started? Are you using a loop device as your PV? Are you using stable versions of lvm2 and device-mapper?