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    Lab 34.4. Recovering from GPT

    Posts: 51
    edited November 2024 in LFS207 Class Forum

    Hi,

    My Ubuntu VM is setup with a GPT partition table and LVM. This is what my system partitions look like:

    1. alu@ubuntu-dt-vm:~$ lsblk | grep -v loop
    2. NAME                      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
    3. sr0                        11:0    1   5.8G  0 rom  /media/alu/Ubuntu 24.04.1 LTS amd64
    4. vda                       253:0    0   128G  0 disk 
    5. \u251c\u2500vda1                    253:1    0     1M  0 part 
    6. \u251c\u2500vda2                    253:2    0     2G  0 part /boot
    7. \u2514\u2500vda3                    253:3    0   126G  0 part 
    8.   \u2514\u2500ubuntu--vg-ubuntu--lv 252:0    0   126G  0 lvm  /

    So I made backups form the GPT configuration from either fdisk and also gdisk. I saved the configs to a USB stick, as I could not access the backups anymore after destroying the GPT patition table.

    So I destroyed the GPT by creating a new table in gdisk without any partitions. I'm not sure that was the right way to go though. After rebooting into the install media, I was able to recover the GPT partition table with the backups, but the system did not boot anyway. However, the partitions are back and I can mount and access the original root filesystem again.

    Any Idea how to proceed with recovering GPT?

    Thanks,
    Urs

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    • Posts: 1,270

      Hi Urs,

      Can you put here all the commands you ran for items 1, 2 and 4, please? I think you used 'vda' instead 'sda', as shown in the guide.

      Regards,
      Luis.

    • Posts: 51

      Hi Luis,

      The vda is the main paravirualized disk of this VM which runs on KVM. It was automatically created like this during the installation of Ubuntu. So there is no sda disk, only this single vda. But the disk type should not matter I guess.

      I did not run either of the commands from step 1, 2 or 4 as these commands are related to the MBR partition (dump of the first couple of bytes on the disk with dd). So I only mad backups of the GPT partition table and destroyed it afterwards. This is what I meant when I mentioned that I don't know if that is the right way to go. Do I have to follow the steps 1, 2 and 4 and dump/destroy/recover the MBR partition with dd even if the VM's disk is configured with GPT?

      Regards,
      Urs

    • Posts: 51

      Sorry, this was the wrong topic. Can you please delete my last post?
      Thanks,
      Urs

    • Posts: 1,270

      Wrong post deleted.

    • Posts: 51

      Hi Luis,

      Thanks for deleting the post. Would be nice, if you could follow up on my last post from November 25.

      Regards,
      Urs

    • Posts: 1,270

      Hi Urs, I see it now.

      So, if you have a GPT partition you need to use the 'sgdisk' tool, as you can see in the man sgdisk(8) page:

      -b, --backup=file
      Save partition data to a backup file. You can back up your current in-memory partition table to a disk file using this option. The resulting file is a binary file consisting of the protective MBR, the main GPT header, the backup GPT header, and one copy of the partition table, in that order. Note that the backup is of the current in-memory data structures, so if you launch the program, make changes, and then use this option, the backup will reflect your changes. If the GPT data structures are damaged, the backup may not accurately reflect the damaged state; instead, they will reflect GPT fdisk's first-pass interpretation of the GPT.

      And then:

      -l, --load-backup=file
      Load partition data from a backup file. This option is the reverse of the -b option. Note that restoring partition data from anything but the original disk is not recommended. This option will work even if the disk's original partition table is bad; however, most other options on the same command line will be ignored.

      From this:

      So I made backups form the GPT configuration from either fdisk and also gdisk.

      I see you already ran gdisk and made a backup, perhaps with something like this, right?

      sgdisk --backup=gptsave

      So, that file generated before by the sgdisk command is the one that you should load with 'sgdisk --load-backup=gspsave' '.

      Regards,
      Luis.

    • Posts: 51

      Hi Luis,

      Thanks, now I got it. Not sure what the issue was the last time. I guess it was just a typo or I forgot to save the restored GPT tables in interactive mode. However, with sgdisk it worked like a charm. I'd suggest to update the according chapter and Lab to cover also the process of backup and recovery of GPT. In case someone will have difficultis too, here are all the steps which I went through:

      1. Backup GPT to a USB thumb drive with sgdisk:
      1. root@ubuntu-dt-vm:~# lsblk
      2. NAME                      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
      3. loop0                       7:0    0     4K  1 loop /snap/bare/5
      4. loop1                       7:1    0  74.3M  1 loop /snap/core22/1564
      5. loop2                       7:2    0  73.9M  1 loop /snap/core22/1663
      6. loop3                       7:3    0 269.8M  1 loop /snap/firefox/4793
      7. loop4                       7:4    0   273M  1 loop /snap/firefox/5273
      8. loop5                       7:5    0  10.7M  1 loop /snap/firmware-updater/127
      9. loop6                       7:6    0  11.1M  1 loop /snap/firmware-updater/147
      10. loop7                       7:7    0 505.1M  1 loop /snap/gnome-42-2204/176
      11. loop8                       7:8    0  91.7M  1 loop /snap/gtk-common-themes/1535
      12. loop9                       7:9    0  10.5M  1 loop /snap/snap-store/1173
      13. loop10                      7:10   0  38.8M  1 loop /snap/snapd/21759
      14. loop11                      7:11   0   500K  1 loop /snap/snapd-desktop-integration/178
      15. loop12                      7:12   0   568K  1 loop /snap/snapd-desktop-integration/253
      16. loop13                      7:13   0  10.7M  1 loop /snap/snap-store/1218
      17. loop14                      7:14   0 150.3M  1 loop /snap/thunderbird/546
      18. loop15                      7:15   0 150.3M  1 loop /snap/thunderbird/562
      19. loop16                      7:16   0  44.3M  1 loop /snap/snapd/23258
      20. sda                         8:0    1  57.3G  0 disk 
      21. \u2514\u2500sda1                      8:1    1  57.3G  0 part /media/alu/ed845a58-3e7c-4652-a256-261e29f1e792
      22. sr0                        11:0    1   5.8G  0 rom  /media/alu/Ubuntu 24.04.1 LTS amd64
      23. vda                       253:0    0   128G  0 disk 
      24. \u251c\u2500vda1                    253:1    0     1M  0 part 
      25. \u251c\u2500vda2                    253:2    0     2G  0 part /boot
      26. \u2514\u2500vda3                    253:3    0   126G  0 part 
      27.   \u2514\u2500ubuntu--vg-ubuntu--lv 252:0    0   126G  0 lvm  /
      28. root@ubuntu-dt-vm:~# 
      29. root@ubuntu-dt-vm:~# 
      30. root@ubuntu-dt-vm:~# sgdisk -b /media/alu/ed845a58-3e7c-4652-a256-261e29f1e792/sgdisk_gpt.bak /dev/vda
      31. The operation has completed successfully.
      32. root@ubuntu-dt-vm:~# ls -l /media/alu/ed845a58-3e7c-4652-a256-261e29f1e792/sgdisk_gpt.bak
      33. -rw-r--r-- 1 root root 17920 Dec  1 11:47 /media/alu/ed845a58-3e7c-4652-a256-261e29f1e792/sgdisk_gpt.bak
      34. root@ubuntu-dt-vm:~# 
      35. root@ubuntu-dt-vm:~# 
      36. root@ubuntu-dt-vm:~# umount /dev/sda1
      37. root@ubuntu-dt-vm:~#
      1. Destroy GPT partition table and reboot
      1. root@ubuntu-dt-vm:~# sgdisk --print-mbr /dev/vda
      2.  
      3. Disk size is 268435456 sectors (128.0 GiB)
      4. MBR disk identifier: 0x00000000
      5. MBR partitions:
      6.  
      7. Number  Boot  Start Sector   End Sector   Status      Code
      8.    1                     1    268435455   primary     0xEE
      9. root@ubuntu-dt-vm:~# 
      10. root@ubuntu-dt-vm:~# 
      11. root@ubuntu-dt-vm:~# sgdisk --print /dev/vda
      12. Disk /dev/vda: 268435456 sectors, 128.0 GiB
      13. Sector size (logical/physical): 512/512 bytes
      14. Disk identifier (GUID): 4472920E-CC02-4ACE-894D-ADBED8562A6E
      15. Partition table holds up to 128 entries
      16. Main partition table begins at sector 2 and ends at sector 33
      17. First usable sector is 2048, last usable sector is 268435422
      18. Partitions will be aligned on 2048-sector boundaries
      19. Total free space is 2015 sectors (1007.5 KiB)
      20.  
      21. Number  Start (sector)    End (sector)  Size       Code  Name
      22.    1            2048            4095   1024.0 KiB  EF02  
      23.    2            4096         4198399   2.0 GiB     8300  
      24.    3         4198400       268433407   126.0 GiB   8300  
      25. root@ubuntu-dt-vm:~#
      26. root@ubuntu-dt-vm:~#
      27. root@ubuntu-dt-vm:~# sgdisk -d 1 /dev/vda
      28. Warning: The kernel is still using the old partition table.
      29. The new table will be used at the next reboot or after you
      30. run partprobe(8) or kpartx(8)
      31. The operation has completed successfully.
      32. root@ubuntu-dt-vm:~# sgdisk -d 2 /dev/vda
      33. Warning: The kernel is still using the old partition table.
      34. The new table will be used at the next reboot or after you
      35. run partprobe(8) or kpartx(8)
      36. The operation has completed successfully.
      37. root@ubuntu-dt-vm:~# sgdisk -d 3 /dev/vda
      38. Warning: The kernel is still using the old partition table.
      39. The new table will be used at the next reboot or after you
      40. run partprobe(8) or kpartx(8)
      41. The operation has completed successfully.
      42. root@ubuntu-dt-vm:~# 
      43. root@ubuntu-dt-vm:~# 
      44. root@ubuntu-dt-vm:~# sgdisk --print /dev/vda
      45. Disk /dev/vda: 268435456 sectors, 128.0 GiB
      46. Sector size (logical/physical): 512/512 bytes
      47. Disk identifier (GUID): 4472920E-CC02-4ACE-894D-ADBED8562A6E
      48. Partition table holds up to 128 entries
      49. Main partition table begins at sector 2 and ends at sector 33
      50. First usable sector is 2048, last usable sector is 268435422
      51. Partitions will be aligned on 2048-sector boundaries
      52. Total free space is 268433375 sectors (128.0 GiB)
      53.  
      54. Number  Start (sector)    End (sector)  Size       Code  Name
      55. root@ubuntu-dt-vm:~#
      56. root@ubuntu-dt-vm:~#
      57. root@ubuntu-dt-vm:~# reboot
      1. Normal boot fails. Hence, boot from rescue media and restore GPT with sgdisk:
      1. root@ubuntu:~# lsblk
      2. NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
      3. loop0    7:0    0   1.6G  1 loop /rofs
      4. loop1    7:1    0   471M  1 loop 
      5. loop2    7:2    0 874.4M  1 loop 
      6. loop3    7:3    0     4K  1 loop /snap/bare/5
      7. loop4    7:4    0  74.3M  1 loop /snap/core22/1564
      8. loop5    7:5    0 269.8M  1 loop /snap/firefox/4793
      9. loop6    7:6    0  10.7M  1 loop /snap/firmware-updater/127
      10. loop7    7:7    0 505.1M  1 loop /snap/gnome-42-2204/176
      11. loop8    7:8    0  91.7M  1 loop /snap/gtk-common-themes/1535
      12. loop9    7:9    0  10.5M  1 loop /snap/snap-store/1173
      13. loop10   7:10   0  38.8M  1 loop /snap/snapd/21759
      14. loop11   7:11   0   500K  1 loop /snap/snapd-desktop-integration/178
      15. loop12   7:12   0 149.6M  1 loop /snap/thunderbird/507
      16. loop13   7:13   0 116.8M  1 loop /snap/ubuntu-desktop-bootstrap/237
      17. sda      8:0    1  57.3G  0 disk 
      18. └─sda1   8:1    1  57.3G  0 part 
      19. sr0     11:0    1   5.8G  0 rom  /cdrom
      20. vda    253:0    0   128G  0 disk 
      21. root@ubuntu:~# 
      22. root@ubuntu:~# 
      23. root@ubuntu:~# mount /dev/sda1 /mnt
      24. root@ubuntu:~# cd /mnt
      25. root@ubuntu:/mnt# ls -l sgdisk_gpt.bak 
      26. -rw-r--r-- 1 root root 17920 Dec  1 10:47 sgdisk_gpt.bak
      27. root@ubuntu:/mnt# 
      28. root@ubuntu:/mnt# 
      29. root@ubuntu:/mnt# sgdisk -l /mnt/sgdisk_gpt.bak /dev/vda 
      30. Warning: The kernel is still using the old partition table.
      31. The new table will be used at the next reboot or after you
      32. run partprobe(8) or kpartx(8)
      33. The operation has completed successfully.
      34. root@ubuntu:/mnt# 
      35. root@ubuntu:/mnt# 
      36. root@ubuntu:/mnt# sgdisk -p /dev/vda
      37. Disk /dev/vda: 268435456 sectors, 128.0 GiB
      38. Sector size (logical/physical): 512/512 bytes
      39. Disk identifier (GUID): 4472920E-CC02-4ACE-894D-ADBED8562A6E
      40. Partition table holds up to 128 entries
      41. Main partition table begins at sector 2 and ends at sector 33
      42. First usable sector is 2048, last usable sector is 268435422
      43. Partitions will be aligned on 2048-sector boundaries
      44. Total free space is 2015 sectors (1007.5 KiB)
      45.  
      46. Number  Start (sector)    End (sector)  Size       Code  Name
      47.    1            2048            4095   1024.0 KiB  EF02  
      48.    2            4096         4198399   2.0 GiB     8300  
      49.    3         4198400       268433407   126.0 GiB   8300  
      50. root@ubuntu:/mnt#
      51. root@ubuntu:/mnt#
      52. root@ubuntu:/mnt# reboot

      So, that's it.

      Thanks again Louis for your support.

      Regards,
      Urs

    • Posts: 1,270

      Hi Urs,

      I'd suggest to update the according chapter and Lab to cover also the process of backup and recovery of GPT.

      Ok, it's a good suggestion, thanks.

      Thanks again Louis for your support.

      It's a pleasure! I'm glad that you are progressing in the course.

      Many regards,
      Luis.

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