By default the system volume group holds only one disk. If you tried to add another disk then you might be received like below error.
Existing space in system volume group
# vgs /dev/cdrom: open failed: No medium found VG #PV #LV #SN Attr VSize VFree applvg 1 1 0 wz--n- 40.68G 692.00M system 1 7 0 wz--n- 19.88G 0
Error while trying to extend the system VG
# vgextend system /dev/sdb1 /dev/cdrom: open failed: No medium found No space for '/dev/sdb1' - volume group 'system' holds max 1 physical volume(s). Unable to add physical volume '/dev/sdb1' to volume group 'system'.
Change the system VG value
# vgchange system -p 0 Volume group "system" successfully changed
Now extending of system VG is successful
# vgextend system /dev/sdb1 Volume group "system" successfully extended
Now you can notice in vgs and pvs that disk has been successully added to VG and PV.
# vgs pv VG #PV #LV #SN Attr VSize VFree applvg 1 1 0 wz--n- 40.68G 692.00M system 2 7 0 wz--n- 29.19G 9.31G
# pvs PV VG Fmt Attr PSize PFree /dev/sda2 system lvm2 a-- 19.88G 0 /dev/sdb1 system lvm2 a-- 9.31G 9.31G /dev/sdb2 applvg lvm2 a-- 40.68G 692.00M