Comment 16 for bug 1895192

OK. I think I finally found the root-cause:

Inside the initramfs /etc/lvm/lvm.conf contains this line:
```
filter = [ "a|/dev/disk/by-id/dm-uuid-.*mpath-.*|", "r|.*|" ]
```

This makes LVM only search the multipath devices for LVM volumes, but in our case the multipath device contains a LUKS container, that contains the LVM volume.
So LVM needs to search the /dev/mapper/dm_crypt-0 block device, which is filtered out by the "r|.*|" regex.

Removing that line from /etc/lvm/lvm.conf (or adopting it accordingly) and running vgchange -a ay makes the ubuntu-vg show up:

```
$ blkid
/dev/mapper/dm_crypt-0: UUID="RP50EB-4DhN-LV9t-9Grx-QSNz-qfK8-lBEY56" TYPE="LVM2
_member"
/dev/mapper/mpatha-part1: UUID="b65d0cc1-0976-48b8-b6bb-e91049700380" BLOCK_SIZE
="4096" TYPE="ext4" PARTUUID="9bc9801d-70b4-42a8-8607-1ca971af476b"
/dev/mapper/mpatha-part2: UUID="b3ebe2f9-10e6-47c0-b30e-256e613eabdd" TYPE="cryp
to_LUKS" PARTUUID="453e1838-94ff-49da-b02e-268a0dabc1de"
/dev/mapper/ubuntu--vg-ubuntu--lv: UUID="e91c7591-b3b0-43db-aad4-092545325799" B
LOCK_SIZE="4096" TYPE="ext4"
/dev/sda: PTUUID="833ca1cf-e90e-4443-b7d3-b41ec73fc35b" PTTYPE="gpt"
/dev/sdc: PTUUID="833ca1cf-e90e-4443-b7d3-b41ec73fc35b" PTTYPE="gpt"
/dev/sde: PTUUID="833ca1cf-e90e-4443-b7d3-b41ec73fc35b" PTTYPE="gpt"
/dev/sdg: PTUUID="833ca1cf-e90e-4443-b7d3-b41ec73fc35b" PTTYPE="gpt"
/dev/mapper/mpatha: PTUUID="833ca1cf-e90e-4443-b7d3-b41ec73fc35b" PTTYPE="gpt"
```

Now let's find out where and why this file is generated.