13 thoughts on - Lvm Snapshot

• What release is that? And what does /proc/cmdline contain when the system boots to the dracut shell?

• sorry: CentOS7 “fresh” minimal with a actual update

  cat proc/cmdline:

  BOOT_IMAGE=/vmlinuz-3.10.0-229.20.1.el7.x86_64
  root=/dev/mapper/CentOS_root ro rd.lvm.lv=CentOS/root
  rd.lvm.lv=CentOS/swap crashkernel=auto rhgb quiet LANG=de_DE.UTF-8
  systemd.debug

  ls /dev/mapper:
  CentOS-swap control

  Zitat von Gordon Messmer :

  Grüße

  Axel

• Creating snapshot:

  [root@lvmtest ~]# lvcreate -L5G -s -n root_snap /dev/CentOS/root
  Reducing COW size 5,00 GiB down to maximum usable size 2,94 GiB.
  Logical volume “root_snap” created.
  [root@lvmtest ~]# lvs
  LV VG Attr LSize Pool Origin Data% Meta% Move
  Log Cpy%Sync Convert
  root CentOS owi-aos— 2,93g
  root_snap CentOS swi-a-s— 2,94g root 0,00
  swap CentOS -wi-ao—- 1,00g

  — reboot —

  dracut shell (missing /dev/CentOS/root)

  cat proc/cmdline:
  BOOT_IMAGE=/vmlinuz-3.10.0-229.20.1.el7.x86_64
  root=/dev/mapper/CentOS_root ro rd.lvm.lv=CentOS/root rd.lvm.lv=CentOS/swap crashkernel=auto rhgb quiet LANG=de_DE.UTF-8
  systemd.debug

  ls /dev/mapper:
  CentOS-swap control

  Grüße

  Axel

• shouldn’t you have a new grub entry with:
  root=/dev/mapper/root_snap rd.lvm.lv=CentOS/root_snap

  Tru

• in journalctl i found:
  modprobe: FATAL: Module dm-snapshot not found
  … Can’t process LV root_snap: snapshot target support missing from kernel

  Zitat von Tru Huynh :

  Grüße

  Axel

• Ok, kernel needs the module dm-snapshot for snapshot support, but the
  modules is storage on a snapshoted-lv …

  you can’t make a root_snapshot with CentOS?

  Zitat von Axel Glienke :

  Grüße

  Axel

• I installed a very simple CentOS 7 system with an LVM root FS. Created a snapshot and rebooted, no problem. Updated dracut, then updated everything else and rebooted, no problem.

  As far as I can tell, dracut normally includes the entire kernel module tree under kernel/drivers/md, which should provide the dm-snapshot module required to complete the boot sequence.

  I can’t determine what’s causing the problem you see, but it doesn’t appear to affect a standard installation.

  Try:

  dracut –force –debug > /var/tmp/dracut.txt 2>&1

  Upload /var/tmp/dracut.txt to http://paste.fedoraproject.org/. I’ll see if I spot anything useful.

• ok, thank you very much Gordon.

  but you know what the difference?

  if i do
  1. CentOS7 installation
  2. yum upgrade
  3. lvcreate -s …
  4. reboot

  then i got the error.

  if i do your way
  1. CentOS7 installtion
  2. snapshot & reboot
  3. yum upgrade
  4. reboot

  then the system start.

  Zitat von Gordon Messmer :

  Grüße

  Axel

• I’ll try to recreate the problem on a new VM, but that doesn’t make much sense. Including the required module isn’t dependent on creating the snapshot as far as I can tell.

  If you can get dracut debug output from an affected system, I might be able to help. Without that, I can’t think of anything else I can offer you.

  dracut –force –debug > /var/tmp/dracut.txt 2>&1

  (Upload /var/tmp/dracut.txt to http://paste.fedoraproject.org/)

• LVM is terribly flawed and most imprudent piece of any filesystem that you can have for snapshot’ing!

  The only snapshot-friendly filesystem or volume manager is ZFS. Whether create a single snapshot or few hundred thousand. It is instant and no data is lost.

• Crazy pants.

  I can reproduce the problem now. I don’t know what the hell is going on, but I’m still looking.

• a manually dracut -f –add-drivers “dm-snapshot”
  solve the problem

  Zitat von Gordon Messmer :

  Grüße

  Axel

• https://bugzilla.redhat.com/show_bug.cgi?id87940

  I’m not sure what I saw that made me think the entire drivers/md tree was included. I deleted the first VM that I used to investigate the problem, so I can’t look at the shell history to figure that one out. In any case, that’s not what dracut does.

  I filed a bug suggesting that dm-snapshot should always be included for systems on LVM.