[bug #31771] GRUB2 with LVM snapshot of /boot

[eugene]

URL:
  <http://savannah.gnu.org/bugs/?31771>

                 Summary: GRUB2 with LVM snapshot of /boot
                 Project: GNU GRUB
            Submitted by: eugeneus
            Submitted on: Fri 03 Dec 2010 02:14:15 PM GMT
                Category: Booting
                Severity: Major
                Priority: 5 - Normal
              Item Group: Software Error
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: Eugene Shtokolov
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 
                 Release: 1.98
         Reproducibility: Every Time
         Planned Release: None

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Details:

Maybe I found a bug or feature in the GRUB2.

I want to create a something like time machine. Install the system on LVM
partition, before the full system upgrade, we take a snapshot. If something
going wrong we can revert the snapshot with command: 
lvconvert --merge /dev/VolGroup00/snap01. 
Of course we can try to boot from LVM snapshot, after a little editing of
grub.cnf. For correct work of this mechanism, we must install the system,
including /boot, on LVM partition. This should work in theory. In practice,
after creating a snapshot the system becomes unbootable. After removing
snapshot the system becomes bootable again.
If I use a separate non LVM partition for /boot, everything works, but if the
kernel has changed, we can't restore system by merging the snapshot.

Sources of information:
https://wiki.archlinux.org/index.php/GRUB2#Installing_GRUB2_during_Arch_Linux_installation
https://wiki.archlinux.org/index.php/LVM#Important

----

Steps to reproduce:
* Boot from archlinux liveCD 2010.05
* modprobe dm-mod
* Create single large physical LVM volume and smaller size logical volume (we
need some free space on the physical volume to create a snapshot):
        cfdisk /dev/sda
        pvcreate /dev/sda1
        pvdisplay 
                  --- Physical volume ---
                  PV Name               /dev/sda1
                  VG Name               VolGroup00
                  PV Size               15.99 GiB / not usable 2.74 MiB
                  Allocatable           yes 
                  PE Size               4.00 MiB
                  Total PE              4094
                  Free PE               1534
                  Allocated PE          2560
                  PV UUID               MZgGF5-XnPs-hzFM-0eWw-mEYP-6wgn-AggDO0
        vgcreate VolGroup00 /dev/sda1
        vgdisplay
              --- Volume group ---
                  VG Name               VolGroup00
                  System ID             
                  Format                lvm2
                  Metadata Areas        1
                  Metadata Sequence No  7
                  VG Access             read/write
                  VG Status             resizable
                  MAX LV                0
                  Cur LV                1
                  Open LV               1
                  Max PV                0
                  Cur PV                1
                  Act PV                1
                  VG Size               15.99 GiB
                  PE Size               4.00 MiB
                  Total PE              4094
                  Alloc PE / Size       2560 / 10.00 GiB
                  Free  PE / Size       1534 / 5.99 GiB
                  VG UUID               1TS4q4-WHAJ-YCxt-RHm0-k0Ha-spS4-O0nZzf 
        lvcreate -L 10G VolGroup00 -n lvolsys
        lvdisplay
                 --- Logical volume ---
                  LV Name                /dev/VolGroup00/lvolsys
                  VG Name                VolGroup00
                  LV UUID                FAfgNj-nDUW-WYyw-wwED-Fmct-it0L-NSOUDt
                  LV Write Access        read/write
                  LV Status              available
                  # open                 1
                  LV Size                10.00 GiB
                  Current LE             2560
                  Segments               1
                  Allocation             inherit
                  Read ahead sectors     auto
                  - currently set to     256
                  Block device           254:0
* Start the installation process: /arch/setup
* Define the mountpoints: lvolsys to /
* Continue with the installation
* Configure the system:
** set USELVM="yes" in /etc/rc.conf
** insert lvm2 in the HOOKS section in /etc/mkinitcpio.conf before
filesystems
** add dm-snapshot to the MODULES variable in /etc/mkinitcpio.conf
* Skip the Install Bootloader step and exit the installer.
* Install the bootloader:
        aif -p partial-configure-network # configure the network
        mount -o bind /dev /mnt/dev
        chroot /mnt bash
        pacman -Sy grub2
        grub-install /dev/sda --no-floppy
        grub-mkconfig -o /boot/grub/grub.cfg
        exit
        reboot

The system boots properly, but it becomes unbootable, after we make snapshot
of the lvolsys volume:
lvcreate --size 100M --snapshot --name snap01 /dev/VolGroup00/lvolsys 
After reboot:
        GRUB loading.
        Welcome to GRUB!
        error: no such disk.
        Entering rescue mode...
        grub rescue> ls
        (hd0) (hd0,1)
If you boot from LiveCD and remove snapshot, the system becomes bootable
again.




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