So you are working with LVM volumes specifically with LVM volumes that contain complete filesystems - i.e. virtual hard disks - and you are getting this 'unknown filesystem type 'LVM2_member' when you are trying to mount the volume. This error is encountered because this partition contain multiple LVM parts (i.e. members). The way you would mount these volumes is to extract this partition, mount it as a loop device and then use pvs/lvs and vgchange command to activate the LVM and mount.
Consider my LVM volume that exist on a Xen/Ganeti host:
# gnt-instance info migrate.isys
[cut]
Disk template: plain
Disks:
- disk 0: lvm, size 10.0G
access mode: rw
logical_id: volg1/50c29778-490a-4686-8a42-569d8b55f40c.disk0_data
on primary: /dev/volg1/50c29778-490a-4686-8a42-569d8b55f40c.disk0_data (254:66)
[cut]
Use kpartx to separate the partitions of the volume:
# kpartx -a -p- /dev/volg1/50c29778-490a-4686-8a42-569d8b55f40c.disk0_data
# ls -la /dev/mapper/volg1-50c29778*
lrwxrwxrwx 1 root root 8 Nov 30 14:46 /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data -> ../dm-66
lrwxrwxrwx 1 root root 8 Nov 30 15:30 /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data-1 -> ../dm-79
lrwxrwxrwx 1 root root 8 Nov 30 15:30 /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data-2 -> ../dm-80
Let's mount the first partition: /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data-1
# mount /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data-1 /mnt
# ls -la /mnt
total 34110
dr-xr-xr-x. 5 root root 1024 Nov 23 13:28 .
drwxr-xr-x 23 root root 4096 May 11 2015 ..
-rw-r--r--. 1 root root 107134 Jul 23 17:13 config-2.6.32-573.el6.x86_64
drwxr-xr-x. 3 root root 1024 Nov 23 13:27 efi
drwxr-xr-x. 2 root root 1024 Nov 23 13:29 grub
-rw-------. 1 root root 27635640 Nov 23 13:29 initramfs-2.6.32-573.el6.x86_64.img
drwx------. 2 root root 12288 Nov 23 13:25 lost+found
-rw-r--r--. 1 root root 205998 Jul 23 17:14 symvers-2.6.32-573.el6.x86_64.gz
-rw-r--r--. 1 root root 2585052 Jul 23 17:13 System.map-2.6.32-573.el6.x86_64
-rwxr-xr-x. 1 root root 4220560 Jul 23 17:13 vmlinuz-2.6.32-573.el6.x86_64
-rw-r--r--. 1 root root 166 Jul 23 17:13 .vmlinuz-2.6.32-573.el6.x86_64.hmac
Looks like it's the /boot partition. Fine, the rest of my disk is on the second partition, so let's mount that:
# mount /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data-2 /mnt
mount: unknown filesystem type 'LVM2_member'
Uh-oh, can not mount: unknown filesystem type 'LVM2_member'. So the second partition must be made up of more than one LVM members. Lets make an image of this and mount it as a loop back filesystem to see what is inside.
# dd if=/dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data-2 of=/var/tmp/partition2.dd bs=1M
10039+0 records in
10039+0 records out
10526654464 bytes (11 GB) copied, 174.754 s, 60.2 MB/s
# file partition2.dd
partition2.dd: LVM2 PV (Linux Logical Volume Manager), UUID: NydiFK-CaAX-PWcz-I0pY-EpAk-4ViC-pIGpf0, size: 10526654464
# losetup /dev/loop0 partition2.dd
# pvs
PV VG Fmt Attr PSize PFree
/dev/loop0 volg0 lvm2 a-- 9.78g 992.00m
/dev/sda5 SYSTEM lvm2 a-- 79.76g 55.55g
/dev/sdb1 volg1 lvm2 a-- 1.97t 1.96t
# lvs
LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
HOME SYSTEM -wi-ao-- 3.72g
ROOT SYSTEM -wi-ao-- 7.45g
SWAP SYSTEM -wi-ao-- 3.72g
TMP SYSTEM -wi-ao-- 1.86g
VAR SYSTEM -wi-ao-- 7.45g
root volg0 -wi----- 7.81g
swap volg0 -wi----- 1.00g
Hey look! It's made up of root and a swap member! Now if you want to mount these, you can now run the vgchange command and activate them:
# vgchange -ay volg0
2 logical volume(s) in volume group "volg0" now active
# lvdisplay
[snip]
--- Logical volume ---
LV Path /dev/volg0/root
LV Name root
VG Name volg0
LV UUID dAoIl6-WR4G-Z0d1-9Cgg-hevk-oSaf-7TTeGv
LV Write Access read/write
LV Creation host, time migrate.isys.bris.ac.uk, 2015-11-23 13:25:26 +0000
LV Status available
# open 0
LV Size 7.81 GiB
Current LE 250
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:5
--- Logical volume ---
LV Path /dev/volg0/swap
LV Name swap
VG Name volg0
LV UUID L0haeP-jWyr-Gij9-sAhA-8mk6-VGS0-M42Gyl
LV Write Access read/write
LV Creation host, time migrate.isys.bris.ac.uk, 2015-11-23 13:25:30 +0000
LV Status available
# open 0
LV Size 1.00 GiB
Current LE 32
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 254:6
# mount /dev/volg0/root /mnt
# ls /mnt
bin boot dev etc home lib lib64 lost+found media mnt opt proc root sbin selinux srv sys tmp usr var
And there you are, you have access to your root data! Copy it, move it, do anything you want, then perform the clean up-reverse actions:
# umount /mnt
# vgchange -an volg0
0 logical volume(s) in volume group "volg0" now active
# losetup -d /dev/loop0
# kpartx -d -p- /dev/volg1/50c29778-490a-4686-8a42-569d8b55f40c.disk0_data
# ls -la /dev/mapper/volg1-50c29778*
lrwxrwxrwx 1 root root 8 Nov 30 14:46 /dev/mapper/volg1-50c29778--490a--4686--8a42--569d8b55f40c.disk0_data -> ../dm-66
