mount: unknown filesystem type ‘LVM2_member’.

When I try to mount a partition, I get the following error

[root@sysresccd ~]# mount /dev/nvme0n1p3 /mnt
mount: /mnt: unknown filesystem type 'LVM2_member'.
[root@sysresccd ~]# 

This is because the disk is LVM. First of all identify the name of the volume group and logical volume.

vgs
lvs

Example

[root@sysresccd ~]# vgs
  VG #PV #LV #SN Attr   VSize  VFree
  vg   2   3   0 wz--n- <1.82t    0 
[root@sysresccd ~]# lvs
  LV   VG Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root vg -wi-a-----   1.80t                                                    
  swap vg -wi-a----- <15.69g                                                    
  tmp  vg -wi-a-----   1.00g                                                    
[root@sysresccd ~]#

From the above result, we found the server has 3 logical volumes. The biggest volume is root. Other volumes swap and tmp we can ignore. So the device name is

/dev/vg/root

Next run

modprobe dm-mod
lvscan
vgchange -ay

Example

[root@sysresccd ~]# lvscan
  ACTIVE            '/dev/vg/swap' [<15.69 GiB] inherit
  ACTIVE            '/dev/vg/tmp' [1.00 GiB] inherit
  ACTIVE            '/dev/vg/root' [1.80 TiB] inherit
[root@sysresccd ~]# vgchange -ay
  3 logical volume(s) in volume group "vg" now active
[root@sysresccd ~]#

Now you should be able to mount with disk using command

mount /dev/vg/root /mnt/