How to make logical volumes on loopback block device available?

Question

I have a file hdd.img being raw image of a hard disk. The harddisk has a BIOS/DOS partition table and its second partition is setup to serve as a the one single physical volume from which LVM creates logical volumes.

My goal is to access the partitions/filesystems that are represented by those logical volumes.

I setup loopback block device mapping for the hdd.img via the command:

$ sudo losetup --partscan -f hdd.img

which results me seeing the two partitions contained in that image on the loopback device (it was loop17)

$ lsblk | grep 'loop17'
loop17                7:17   0  30G  0 loop
├─loop17p1          259:4    0   1G  0 loop
└─loop17p2          259:5    0  29G  0 loop

The question now is. How can I inform the system to scan /dev/loop17p2 for the logical volumes I actually want to access

Update

Installing the package kpartx and doing this:

$ sudo losetup -f hdd.img
$ sudo kpartx -a /dev/loop17

did generate the lvm mappings to the volumes contained on /dev/loop17p2. While this solves my immediate needs it would yet still be great to know why vgscan does not work once the losetup --partscan -f hdd.img had already created the /dev/loop17p2 block device that contains the logical volumes?

Answer 1

Use lvscan. vgscan is to scan Volume Group ...

Answer 2

The command vgscan has an command line option --cache that must be used to put the the results of the scan into the cache, allowing a subsequent vgchange -ay <vg> call to genrete the mapping

In short this work:

$ sudo losetup --partscan -f hdd.img
$ sudo vgscan --scan
$ sudo vgchange -ay <vg>

whith vg being the one of the volume group (which the vgscan) has reported.

This would however not work:

$ sudo losetup --partscan -f hdd.img
$ sudo vgscan
$ sudo vgchange -ay <vg>

because the second command would have not put its scanning results into the --cache. Oddly enough my take on the --cache flag was to think it meant to use cached values instead of performing an actual scan.