Arch Linux

eugeneus

Member

Registered: 2010-11-27

Posts: 3

GRUB2 with LVM snapshots

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/GR … stallation
https://wiki.archlinux.org/index.php/LVM#Important

Steps to reproduce:

1. Boot from archlinux liveCD 2010.05

2. modprobe dm-mod

3. 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 

4. Start the installation process: /arch/setup

5. Define the mountpoints: lvolsys to /

6. Continue with the installation

7. Configure the system:

   1. set USELVM="yes" in /etc/rc.conf

   2. insert lvm2 in the HOOKS section in /etc/mkinitcpio.conf before filesystems

   3. add dm-snapshot to the MODULES variable in /etc/mkinitcpio.conf

8. Skip the Install Bootloader step and exit the installer.

9. 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 

If you boot from LiveCD and remove snapshot, the system becomes bootable again.

Please confirm this bug and I'll report on https://bugs.archlinux.org

eugeneus

Member

Registered: 2010-11-27

Posts: 3

Re: GRUB2 with LVM snapshots

Small addition. GRUB output:

GRUB loading.
Welcome to GRUB!
error: no such disk.
Entering rescue mode...
grub rescue> ls
(hd0) (hd0,1)

ArchIMEDE

Member

Registered: 2010-04-20

Posts: 14

Re: GRUB2 with LVM snapshots

Could you please attach contents of grub.cfg?

eugeneus

Member

Registered: 2010-11-27

Posts: 3

Re: GRUB2 with LVM snapshots

I report this bug to the GNU GRUB bugtracker (http://savannah.gnu.org/bugs/?group=grub) and receive a message:
This was fixed in 1.99
Please mark this topic as FIXED

Last edited by eugeneus (2010-12-07 09:03:05)