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

Mon Jun 20 23:13:21 UTC 2016

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.

Chris

On 06/20/2016 04:43 PM, Chris Friesen wrote:
> Found it.  /usr/lib/udev/rules.d/11-dm-lvm.rules is what makes the
> /dev/<VG>/<LV> symlink for normal devices, but for a thin pool with a thin
> volume in it it will exit without making a symlink because
> DM_UDEV_DISABLE_SUBSYSTEM_RULES_FLAG=1 is set.
>
> Given that both vgmknodes and the udev rules come from the lvm2 package, it'd be
> nice if they agreed on whether or not a symlink should be created and if so
> where it should link to.
>
> Chris
>
>
> On 06/20/2016 03:53 PM, Chris Friesen wrote:
>> If I run it I get this:
>>
>> controller-1:/dev# vgmknodes -v
>>      Using logical volume(s) on command line.
>>      Found same device /dev/sda6 with same pvid Fw6C1IZABbnspIT23RbOGf2DXuB4zMhS
>>      Found same device /dev/sda5 with same pvid 1s1dPDodojAS0kqToIRy4hiXFjCp2t2o
>>      Found same device /dev/sda6 with same pvid Fw6C1IZABbnspIT23RbOGf2DXuB4zMhS
>>      Found same device /dev/drbd4 with same pvid 6yk0HVTSbU9Amo6mwPKt3nZPnBIXBlhR
>>      Found same device /dev/sda5 with same pvid 1s1dPDodojAS0kqToIRy4hiXFjCp2t2o
>>    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.
>>
>>
>> So it thinks that udev should have already made it.  Also, the symlink it
>> generates looks different than the symlinks already in /dev/cinder-volumes, in
>> that it points to /dev/mapper/<name> rather than pointing to the "real"
>> /dev/dm-X device like the others.
>>
>>
>> controller-1:/dev# ls -l /dev/cinder-volumes/
>> total 0
>> lrwxrwxrwx 1 root root  7 Jun 20 17:57 anchor-lv -> ../dm-9
>> lrwxrwxrwx 1 root root 49 Jun 20 21:48 cinder-volumes-pool ->
>> /dev/mapper/cinder--volumes-cinder--volumes--pool
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-0bc1df18-45d0-4477-9c57-36876d3f82d4 -> ../dm-19
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-2fff261f-8860-4b86-8b2e-49bddcf47e9b -> ../dm-17
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-48744604-6b02-4f11-ba02-3f692d109953 -> ../dm-15
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-8dabc793-e46d-4849-a2fb-dd3d4bc2c988 -> ../dm-20
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-be3c9ddb-a6eb-43ca-ac37-5554756a4c13 -> ../dm-16
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-eef89318-fa8e-4ca2-a8a7-fe8e143d8792 -> ../dm-14
>> lrwxrwxrwx 1 root root  8 Jun 20 17:57
>> volume-f11a7d88-1ad3-4e89-a594-e824019725bb -> ../dm-18
>>
>>
>> Given the above, I think that something other than vgmknodes must be involved.
>>
>> Chris
>>
>>
>> On 06/20/2016 03:03 PM, Ilya Boka wrote:
>>> Command vgmknodes
>>>
>>> On Mon, Jun 20, 2016 at 10:52 PM, Chris Friesen
>>> <chris.friesen at windriver.com> wrote:
>>>> Hi,
>>>>
>>>> Can someone tell me what creates the /dev/<volgroup>/<volume symlinks?  Is
>>>> this LVM or udev (and if udev, do you know which rule)?
>>>>
>>>> I'm seeing some interesting behaviour where if I create a thin pool it
>>>> creates a symlink for the pool, but once I create a thin volume within the
>>>> pool then the pool symlink disappears.
>>>>
>>>> Thanks,
>>>> Chris
>>>>
>>>> _______________________________________________
>>>> linux-lvm mailing list
>>>> linux-lvm at redhat.com
>>>> https://www.redhat.com/mailman/listinfo/linux-lvm
>>>> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>>>
>>> _______________________________________________
>>> linux-lvm mailing list
>>> linux-lvm at redhat.com
>>> https://www.redhat.com/mailman/listinfo/linux-lvm
>>> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>>>
>>
>