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How to mount a lvm volume which is inside a lvm volume

Julian
see https://dev.to/c33s/comment/3eck
・3 min read

if you use something like kvm for virtualization and also use lvm on the host system a good practice is to create a volume for each virtual machine.

to don't make a difference between a physical system and a virtual system, i like to also use lvm in the virtual systems. it works like charm but mounting a volume from virtual machine is kind of tricky. it's getting even trickier if your volume groups all have the same names.

kvm host:
    vg_main:
        home
        tmp
        ...
        kvm_web01
            vg_main:
                home
                tmp
                ...

so if you try to mount kvm_web01 directly you get an error:

mount /dev/mapper/vg_main-kvm_web01  /mnt/web01/
mount: /mnt/web01: wrong fs type, bad option, bad superblock on /dev/mapper/vg_main-kvm_web01, missing codepage or helper program, or other error.

with kpartx (aptitude install kpartx) you can detect partitions inside of an lvm volume and make them accessible:

kpartx -av /dev/mapper/vg_main-kvm_web01
add map vg_main-kvm_web01p1 (253:16): 0 389120 linear 253:7 2048
add map vg_main-kvm_web01p2 (253:17): 0 2 linear 253:7 393214
add map vg_main-kvm_web01p5 (253:18): 0 83490816 linear 253:7 393216

now the partitions are accessible. you can mount the non lvm boot partition vg_main-kvm_web01p1 but you still get an error if you try to mount the physical volume on partition vg_main-kvm_web01p2

mount: ....  unknown filesystem type 'LVM2_member'.

(this message is not 100% accurate because i already finished the recovery and i am reproducing the steps without redoing all again)

with pvscan the partition made visible with kpartx are now also known by lvm.

pvscan
vgdisplay

in my case i ran in the next problem of having two volume groups with the same name vg_main. lvs gave the correct output with most of the partitions duplicated

lvs
root@zhaan ~ # lvs
  LV             VG      Attr     LSize   Pool Origin Data%  Move Log Copy%  Convert
  home           vg_main -wi-ao--   1.00g
  home           vg_main -wi-ao--   2.00g
...

so you use vgdisplay to find out the id of the inner volume group (should be the one with less harddisk space) and vgrename to rename it.

vgrename -v 6FW1tp-aeVy-1oTW-Ho1M-DwV7-TPB0-QCIoz4 vg_web01
    Cache: Duplicate VG name vg_main: Existing 306MGq-WfN8-LFx5-SKvw-uIfv-dSj0-QCSfGT (created here) takes precedence over 6FW1tp-aeVy-1oTW-Ho1M-DwV7-TPB0-QCIoz4
  Processing VG vg_main because of matching UUID 6FW1tp-aeVy-1oTW-Ho1M-DwV7-TPB0-QCIoz4
    Cache: Duplicate VG name vg_main: Existing 306MGq-WfN8-LFx5-SKvw-uIfv-dSj0-QCSfGT (created here) takes precedence over 6FW1tp-aeVy-1oTW-Ho1M-DwV7-TPB0-QCIoz4
    Archiving volume group "vg_main" metadata (seqno 8).
    Writing out updated volume group
    Renaming "/dev/vg_main" to "/dev/vg_web01"
    Creating volume group backup "/etc/lvm/backup/vg_web01" (seqno 9).
  Volume group "6FW1tp-aeVy-1oTW-Ho1M-DwV7-TPB0-QCIoz4" successfully renamed to "vg_web01"

and the use vgchange to "refresh" the logical volume list:

vgchange -a y
  6 logical volume(s) in volume group "vg_web01" now active
  16 logical volume(s) in volume group "vg_main" now active

then you simply use mount to make it accessible on your host system:

mount /dev/vg_web01/home /mnt/web01/home

lvm rocks! :)

(hope i put my notes together correct. if you find an error trying it out, please comment)

links:

Discussion (2)

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keithpjolley profile image
keith p. jolley

Well done - thank you!

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arul1712 profile image
arul1712

is that possible to snapshot inside a lvm ? is that any way to mount that snapshot inside a lvm?