[linux-lvm] what creates the symlinks in /dev/<volgroup> ?

Chris Friesen chris.friesen at windriver.com
Tue Jun 21 15:22:44 UTC 2016

I'm using the stock CentOS7 version, I think.

   LVM version:     2.02.130(2)-RHEL7 (2015-12-01)
   Library version: 1.02.107-RHEL7 (2015-12-01)
   Driver version:  4.33.0

So are you saying that nobody should run "vgscan --mknodes" on a system where 
udev is managing the symlinks?

Personally, I think that running that command should use the same logic as the 
udev rules to decide whether or not to create a symlink, and should create the 
symlink pointing to the same place as the udev rules.

I'm not sure what you're talking about as far as duplicates, I'm not seeing any 
duplicate devices.  The problem I see is that "vgscan --mknodes" or "vgmknodes" 
will both create an additional symlink when compared to the udev rules, and the 
additional symlink is not deleted when I deactivate the volume group.

For what it's worth, in my lvm.conf I have "use_lvmetad = 0" and

global_filter = [ "a|/dev/sda|", "a|/dev/drbd4|",  "a|/dev/sda6|", "r|.*|" ]


On 06/21/2016 03:07 AM, Zdenek Kabelac wrote:
> Dne 21.6.2016 v 01:13 Chris Friesen napsal(a):
>> It appears that "vgscan --mknodes" also creates this symlink, and creates it
>> pointing to /dev/mapper/<name> instead of to /dev/dm-X like udev does.
>> controller-1:/dev# vgscan -v --mknodes
>>     Wiping cache of LVM-capable devices
>>     Wiping internal VG cache
>>   Reading all physical volumes.  This may take a while...
>>     Using volume group(s) on command line.
>>   Found volume group "nova-local" using metadata type lvm2
>>   Found volume group "cgts-vg" using metadata type lvm2
>>   Found volume group "cinder-volumes" using metadata type lvm2
>>     Using logical volume(s) on command line.
>>   The link /dev/cinder-volumes/cinder-volumes-pool should have been created by
>> udev but it was not found. Falling back to direct link creation.
>> controller-1:/dev# ls -l /dev/cinder-volumes/
>> total 0
>> lrwxrwxrwx 1 root root  7 Jun 20 22:43 anchor-lv -> ../dm-9
>> lrwxrwxrwx 1 root root 49 Jun 20 23:09 cinder-volumes-pool ->
>> /dev/mapper/cinder--volumes-cinder--volumes--pool
>> This is a bad thing, because running the above or "vgmknodes" and then running
>> "vgchange -an cinder-volumes" will leave the /dev/cinder-volumes directory
>> with a dangling symlink in it.
>> This in turn breaks /usr/lib/ocf/resource.d/heartbeat/LVM, which (perhaps
>> erroneously) uses the existence of a non-empty /dev/<volume_group> directory
>> as a test to see if the volume group is active or not.
> This whole thread is presentint  one continues sets of bugs and misunderstandings.
> On modern Linux system - it should be ONLY udev to create any links in /dev dir.
> So  lvm2 does not create ANY links unless forced to do.
> So the clear answer here is   'it is udev rule that creates links'
> Now the second part - you system is likely misconfigured.
> It's admin responsibility to filter out devices in a way lvm2 does not get any
> duplicates.  If the same device (same UUID) is seen multiple times,
> lvm2 has no way to know which of them is the right one to use.
> Since you have not even shown the version of lvm2 in use and whather lvmetad is
> in use - there is no way to give you any more hints...
> Regards
> Zdenek
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