6. Metadata Recover of a Physical Volume

April 10, 2020 Logical Volume Manager

< 5 LVM Snapshots | Commands | 7 LVM Attributes >

33 & 34. Recovering a Faulty Disk

https://www.udemy.com/course/a-complete-guide-on-linux-lvm/learn/lecture/13062490#overview

Overview
- It is possible for the volume group metatdata area of a physical volume to be over-written or deleted.
- An error message is displayed indicating the metadata area is incorrect or the system was unable to find a physical volume with the particular UUID.
Symptoms
- lvs -a -o +devices
  - Couldn’t find device with uuid ‘gobblety-gook….’
  - Couldn’t find all physical volumes for volume group VG

34 Lab

https://www.udemy.com/course/a-complete-guide-on-linux-lvm/learn/lecture/13062660#overview

# view # of PV in a VG
vgs
  VG   #OV  #LV  #SN ...
  VG   3    1    0
  ...
# display information about the PEs
vgdisplay -v /dev/VG
  ...
  --- Physical Volumes ---
  PV Name  /dev/sdg1
  PV UUID  AOSIDq-alE5-...
  PV Status  allocatable
  Total PE / Free PE  511 / 255

  PV Name /dev/sdh1
  PV UUID B1SmDq-blEy-...
  PV Status allocatable
  Total PE / Free PE 511 / 255

  PV Name /dev/sdi1
  PV UUID C2SIkq-clEp-...
  PV Status allocatable
  Total PE / Free PE 511 / 255

Purposefully ruin a drive’s metadata

dd if=//dev/zero of=/dev/sdg1 bs=1024 count=1
# Look at the info again
vgdisplay -v /dev/VG
...
--- Physical Volumes ---
  PV Name unknown device
  PV UUID AOSIDq-alE5-...
  PV Status allocatable
  Total PE / Free PE 511 / 255
lvs -a -o +devices
  WARNING: Device for PV AOSIDq-alE5-... not found or rejected by a filter
  ...
  data LV -wi-a---p- 1.00g
  unknown device(0)

How to correct

# First install a new disk
#
# Partially unmount the filesystem
vgchange -an --partial
# Update the new disk with the old UUID and copy the data
pvcreate --uuid "AOSIDq-alE5-..." --restorefile /etc/lvm/backup/VG /dev/sdg1
# Restore the remaining metadata
vgcfgrestore /dev/VG
# Activate the drive
vgchange -a y /dev/VG
# Verify everything is working
vgdisplay -v /dev/VG
  ...
  --- Physical Volumes ---
  PV Name /dev/sdg1
  PV UUID AOSIDq-alE5-...
  PV Status allocatable
  Total PE / Free PE 511 / 255

Using the LVM backup file

# change to the LVM backup directory
sudo su
cd /etc/lvm/backup/VG
# Show your volume groups
vgs
  VG    #PV #LV #SN Attr   VSize   VFree
  myVG    1   1   0 wz--n- 196.96g    0  
# edit that file
cat myVG

Actual File data from my lab

# Generated by LVM2 version 2.02.180(2)-RHEL7 (2018-07-20): Mon Apr  6 17:42:57 2020

contents = "Text Format Volume Group"
version = 1

description = "Created *after* executing 'lvcreate -n cc138e33-c171-6d3f-e559-f9649d47ad75 -L 201688 XSLocalEXT-cc138e33-c171-6d3f-e559-f9649d47ad75'"

creation_host = "citrix01"	# Linux citrix01 4.19.0+1 #1 SMP Wed Dec 4 13:59:08 UTC 2019 x86_64
creation_time = 1586212977	# Mon Apr  6 17:42:57 2020

XSLocalEXT-cc138e33-c171-6d3f-e559-f9649d47ad75 {
	id = "PQfRRm-HJvu-2L3r-ohPX-tWaj-azeD-mwIaxj"
	seqno = 2
	format = "lvm2"			# informational
	status = ["RESIZEABLE", "READ", "WRITE"]
	flags = []
	extent_size = 8192		# 4 Megabytes
	max_lv = 0
	max_pv = 0
	metadata_copies = 0

	physical_volumes {

		pv0 {
			id = "14dfcf-uhNt-wav7-d1jH-hdOH-fGCr-nABNQi"
			device = "/dev/sda3"	# Hint only

			status = ["ALLOCATABLE"]
			flags = []
			dev_size = 413084303	# 196.974 Gigabytes
			pe_start = 22528
			pe_count = 50422	# 196.961 Gigabytes
		}
	}

	logical_volumes {

		cc138e33-c171-6d3f-e559-f9649d47ad75 {
			id = "PpOE7h-qUkF-grNR-2zxi-GMZC-S9HQ-mKhJY6"
			status = ["READ", "WRITE", "VISIBLE"]
			flags = []
			creation_time = 1586212977	# 2020-04-06 17:42:57 -0500
			creation_host = "citrix01"
			segment_count = 1

			segment1 {
				start_extent = 0
				extent_count = 50422	# 196.961 Gigabytes

				type = "striped"
				stripe_count = 1	# linear

				stripes = [
					"pv0", 0
				]
			}
		}
	}

}

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