#3043 PR `merged`: Remove the lvmdevices file at the end of recovery¶

Labels: enhancement, bug, fixed / solved / done

pcahyna opened issue at 2023-08-24 14:17:¶

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Pull Request Details:¶

Type: Bug Fix
Impact: High
Reference to related issue (URL): closes #2917, closes #3042
How was this pull request tested?
Full backup and recovery on a KVM virtual machine (x86_64) with virtio disk (vda) and a PowerVM LPAR (ppc64le) withs SCSI disk (sda), both with RHEL 9 (where the LVM default is use_devicesfile=1).
Before backup and recovery:

# cat /etc/lvm/devices/system.devices
# LVM uses devices listed in this file.
# Created by LVM command lvmdevices pid 3645 at Thu Aug 24 11:52:38 2023
VERSION=1.1.4
IDTYPE=sys_serial IDNAME=00fac60100004c000000017bead7ca41.10 DEVNAME=/dev/sda3 PVID=3NFhwAW1RAQzJz8uOsZqkcqasKhlZt9b PART=3
# lvmdevices 
  Device /dev/sda3 IDTYPE=sys_serial IDNAME=00fac60100004c000000017bead7ca41.10 DEVNAME=/dev/sda3 PVID=3NFhwAW1RAQzJz8uOsZqkcqasKhlZt9b PART=3

After recovery:

# lvmdevices 
  Devices file does not exist.
# cat /etc/lvm/devices/system.devices.rearbak 
# LVM uses devices listed in this file.
# Created by LVM command lvmdevices pid 3645 at Thu Aug 24 11:52:38 2023
VERSION=1.1.4
IDTYPE=sys_serial IDNAME=00fac60100004c000000017bead7ca41.10 DEVNAME=/dev/sda3 PVID=3NFhwAW1RAQzJz8uOsZqkcqasKhlZt9b PART=3

Brief description of the changes in this pull request:

The file /etc/lvm/devices/system.devices restricts LVM to disks with given (hardware) IDs (serial numbers, WWNs). See lvmdevices(8).

Unfortunately, when restoring to different disks than in the original system, it will mean that LVM is broken in the recovered system (it won't find any disks). Therefore it is safer to remove the file to force the old behavior where LVM scans all disks. This used to be the LVM default (use_devicesfile=0).

I don't think one can delete the devices file and do /usr/sbin/vgimportdevices -a to reimport the VGs automatically at the end of recovery. The problem is, imagine you have several volume groups. One for the system (rootvg) and one for data (datavg), on different disks. Now you system is broken and you want to recover it, but the data are unaffected and you want them to survive the recovery procedure. For this, it is advisable to disconnect the data disks during the recovery procedure to avoid accidentally reformatting them. But, as they are not connected, /usr/sbin/vgimportdevices -a will not find datavg, therefore it will not add them to the devices file and when you reboot after recovery with the disks attached again, they will not be found by LVM and datavg will not be activated.
I was thinking about some automated procedure to update only the obsolete entries in lvmdevices and keep all others, but I have not found anything simple. I therefore believe the only reliable solution is to remove the devices file at the end of recovery, since running without it is preferable to a broken system due to some fragile attempt at an "ideal" solution.

pcahyna commented at 2023-08-24 14:23:¶

One might be tempted to run /usr/sbin/vgimportdevices -a without removing the devices file, as it leaves the old entries in the file and merely adds new ones, and thus does not suffer from the problem above. However, running the command when there already are entries for the newly added PVs that point to unknown disks, vgimportdevices asks interactively about it:

  WARNING: adding device /dev/sdd with PVID iinTjXXywDCXcqssEPsSM8HvuFdWh1OX which is already used for missing device device_id 00fac60100004c000000017bead7ca41.70.
Add device with duplicate PV to devices file?

and if one confirms it, every LVM command then complains about duplicate PV entries in the file:

pvdisplay 
  Devices file sys_serial 00fac60100004c000000017bead7ca41.70 PVID iinTjXXywDCXcqssEPsSM8HvuFdWh1OX last seen on /dev/sdc not found.

pcahyna commented at 2023-08-24 14:28:¶

@rear/contributors I would like to merge the PR quite soon, if there is nothing obviously wrong with it. I believe the approach is safe, even if not ideal (we are reverting to the old behavior without devices file) and after much thinking I have not found a better approach.

pcahyna commented at 2023-08-24 16:09:¶

Relevant part of console output:

Created SELinux /mnt/local/.autorelabel file : after reboot SELinux will relabel all files 
Recreating directories (with permissions) from /var/lib/rear/recovery/directories_permissions_owner_group 
Checking if certain restored files are consistent with the recreated system 
Renamed LVM devices file /mnt/local//etc/lvm/devices/system.devices to /mnt/local//etc/lvm/devices/system.devices.rearbak 
to prevent LVM problems in the recovered system, verify that the file 
is correct after booting the recovered system and move it back, or 
regenerate it using vgimportdevices. 
Migrating disk-by-id mappings in certain restored files in /mnt/local to current disk-by-id mappings ... 
Running dracut...

[Export of Github issue for rear/rear.]

#3043 PR merged: Remove the lvmdevices file at the end of recovery¶