Why are VG tags not visible from another node when using LVM on shared storage?

Solution Unverified - Updated

Environment

  • Red Hat Enterprise Linux 7 - non-clustered environment

Issue

  • Having a shared storage without cluster with LVM makes the VG tags created on one node not visible from the another node

Resolution

To force reading metadata from disk instead of cache use the '--cache' argument
for commands 'pvscan' or 'vgscan'. 

pvscan --cache

vgscan --cache

Alternatively restart the LVM metadata caching service.

systemctl restart lvm2-lvmetad.service

Root Cause

The reason of not seeing metadata changes on another node is that by default
there is 'lvm2-lvmetad.service' service running.
This service is caching the LVM metadata which also includes the tags.

man 8 pvscan

  When lvmetad is used, LVM commands avoid scanning disks  by  reading  metadata  from  lvmetad.
  When  new disks appear, they must be scanned so their metadata can be cached in lvmetad.  This
  is done by the command pvscan --cache, which scans disks and passes the metadata to lvmetad.

  The pvscan --cache command is typically run automatically by system services when a new device
  appears.   Users  do not generally need to run this command if the system and lvmetad are run‐
  ning properly.

  Many scripts contain unnecessary pvscan (or vgscan) commands for historical reasons.  To avoid
  disrupting the system with extraneous disk scanning, an ordinary pvscan (without --cache) will
  simply read metadata from lvmetad like other LVM commands.  It does  not  do  anything  beyond
  displaying the current state of the cache.

Diagnostic Steps

Having two nodes that are sharing common storage with LVM without clustering

  • On node1 adding tag to VG
[root@node1]# vgchange --addtag newtag testvg
[root@node1]# vgs -o+vg_tags testvg
  VG #PV #LV #SN Attr   VSize  VFree  VG Tags 
  testvg   1   1   0 wz--n- 10.00g  6.00g newtag
  • On node2 checking if tag exists
[root@node2]# vgs -o+vg_tags testvg
  VG #PV #LV #SN Attr   VSize  VFree  VG Tags 
  testvg   1   1   0 wz--n- 10.00g  6.00g
  • running commands 'vgscan' and 'pvscan' doesn't help and tag is not visible on the node2
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