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I am trying to create a snapshot of a logical volume (partition) but I cannot make it work. After days of researching, I found that this post might shed some light on the situation. We are using the Yocto project for the basis of our OS.

I am using lvcreate -L 8M -n conf_snap -s /dev/mapper/l4m-conf -v and this generates the following:

Overriding default 'mirror_region_size' setting with 'raid_region_size' setting of 2048 kiB
Setting chunksize to 4.00 KiB.
Archiving volume group "l4m" metadata (seqno 15).
Creating logical volume conf_snap
Creating volume group backup "/etc/lvm/backup/l4m" (seqno 16).
activation/volume_list configuration setting not defined: Checking only host tags for l4m/conf_snap.
Creating l4m-conf_snap
Loading table for l4m-conf_snap (252:8).
Resuming l4m-conf_snap (252:8).
/dev/l4m/conf_snap: not found: device not cleared
Aborting. Failed to wipe snapshot exception store.
Removing l4m-conf_snap (252:8)
Creating volume group backup "/etc/lvm/backup/l4m" (seqno 17).
  1. systemd-udevd.service is up and running
  2. vigdisplay outputs
 --- Volume group ---
 VG Name               l4m
 System ID
 Format                lvm2
 Metadata Areas        2
 Metadata Sequence No  17
 VG Access             read/write
 VG Status             resizable
 MAX LV                0
 Cur LV                7
 Open LV               6
 Max PV                0
 Cur PV                2
 Act PV                2
 VG Size               952.85 GiB
 PE Size               4.00 MiB
 Total PE              243930
 Alloc PE / Size       236533 / <923.96 GiB
 Free  PE / Size       7397 / 28.89 GiB
 VG UUID               A2tiw5-tF9k-pIjQ-a9W5-4LbN-LcR3-mVMzCi
  1. lsblk outputs
NAME             MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                8:0    0 924.1G  0 disk
|-sda1             8:1    0   100M  0 part
`-sda2             8:2    0   924G  0 part
  `-l4m          252:0    0   924G  0 crypt
    |- (...)
    |-l4m-conf   252:4    0    20M  0 lvm   /mnt/conf
    |- (...)
sdb                8:16   0  14.9G  0 disk
| (...)
sdc                8:32   1  28.9G  0 disk

The sdc device is a USB storage device that I added to VG using pvcreate and vgextend.

2

Well, this is nice, posting an answer to your own bountied question. When I say nice, of course I mean not.

The reason why LVM fails to create the snapshot volume is because of a glitch in udev version 239 and 240 discussed here. There it says that LVM volumes are not properly initialized by the udev. When this is put together with information found in this thread on RedHat narkive, it seems pretty clear that this error is due to the udev version.

To fix this I would need to update my udev, but since it comes bundled in systemd of OpenEmbedded this is not an option at this time.

I hope this helps someone.

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