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7.8. Duplicate PV Warnings for Multipathed Devices

When using LVM with multipathed storage, some LVM commands (such as vgs or lvchange) may display messages such as the following when listing a volume group or logical volume. 
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/dm-5 not /dev/sdd
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/emcpowerb not /dev/sde
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/sddlmab not /dev/sdf
After providing information on the root cause for these warnings, this section describes how to address this issue in the following two cases.
  • The two devices displayed in the output are both single paths to the same device
  • The two devices displayed in the output are both multipath maps

7.8.1. Root Cause of Duplicate PV Warning

With a default configuration, LVM commands will scan for devices in /dev and check every resulting device for LVM metadata. This is caused by the default filter in the /etc/lvm/lvm.conf, which is as follows: 
filter = [ "a/.*/" ]
When using Device Mapper Multipath or other multipath software such as EMC PowerPath or Hitachi Dynamic Link Manager (HDLM), each path to a particular logical unit number (LUN) is registered as a different SCSI device, such as /dev/sdb or /dev/sdc. The multipath software will then create a new device that maps to those individual paths, such as /dev/mapper/mpath1 or /dev/mapper/mpatha for Device Mapper Multipath, /dev/emcpowera for EMC PowerPath, or /dev/sddlmab for Hitachi HDLM. Since each LUN has multiple device nodes in /dev that point to the same underlying data, they all contain the same LVM metadata and thus LVM commands will find the same metadata multiple times and report them as duplicates.
These duplicate messages are only warnings and do not mean the LVM operation has failed. Rather, they are alerting the user that only one of the devices has been used as a physical volume and the others are being ignored. If the messages indicate the incorrect device is being chosen or if the warnings are disruptive to users, then a filter can be applied to search only the necessary devices for physical volumes, and to leave out any underlying paths to multipath devices.