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[George Moutsopoulos <1423796@xxxxxxxxxxxxxxxxxx>]

OK, I managed! I had to add more modules

sudo echo "dm_cache" >> /etc/initramfs-tools/modules
sudo echo "dm_cache_mq"  >> /etc/initramfs-tools/modules
sudo echo "dm_persistent_data"  >> /etc/initramfs-tools/modules
sudo echo "dm_bufio"  >> /etc/initramfs-tools/modules

plus, add cache_check as in 
 http://forums.debian.net/viewtopic.php?f=5&t=119644

Booted!

This is how I set up the lvmcache
sudo vgextend LinuxVG /dev/sdb2
sudo lvcreate -L 10G -n rootCacheLV LinuxVG /dev/sdb2
sudo lvcreate -L 24M -n rootCacheMetaLV LinuxVG /dev/sdb2
sudo lvcreate -L 10G -n homeCacheLV LinuxVG /dev/sdb2
sudo lvcreate -L 24M -n homeCacheMetaLV LinuxVG /dev/sdb2
sudo lvconvert --type cache-pool --poolmetadata LinuxVG/rootCacheMetaLV --cachemode writethrough LinuxVG/rootCacheLV
sudo lvconvert --type cache-pool --poolmetadata LinuxVG/homeCacheMetaLV --cachemode writethrough LinuxVG/homeCacheLV
sudo lvconvert --type cache --cachepool LinuxVG/rootCacheLV LinuxVG/rootLV
sudo lvconvert --type cache --cachepool LinuxVG/homeCacheLV LinuxVG/homeLV

where LinuxVG has the root at rootLV and home at homeLV and sdb2 is an
SSD

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https://bugs.launchpad.net/bugs/1423796

Title:
  Unable to mount lvmcache root device at boot time

Status in initramfs-tools package in Ubuntu:
  Confirmed
Status in linux package in Ubuntu:
  Confirmed
Status in lvm2 package in Ubuntu:
  Confirmed

Bug description:
  I'm experimenting with Vivid Vervet on a virtual machine and tried a
  new LVM feature, lvmcache. I made a cache for the root file system,
  rebuilt the initrd and rebooted the VM.

  At boot time, the system failed to activate the root LV. After some
  investigation, I found out, it's because the initrd is missing some
  essential stuff needed for activating a cached LV.

  The initrd was missing the dm-cache module. I regenerated the initrd
  with explicitly listing dm-cache in /etc/initramfs-tools/modules, but
  the system still can't boot up, because now it is missing the
  /usr/sbin/cache_check utility.

  As SSDs are becoming more and more common, I think it will be common
  to use them as cache for root file systems, thus it is mandatory to
  make sure that an initrd can mount an lvmcached root device when
  necessary, preferably without /etc/initramfs-tools/modules and other
  manual initrd hacking.

  System details:

  Linux lvmvm 3.18.0-13-generic #14-Ubuntu SMP Fri Feb 6 09:55:14 UTC
  2015 x86_64 x86_64 x86_64 GNU/Linux

  Distributor ID:	Ubuntu
  Description:	Ubuntu Vivid Vervet (development branch)
  Release:	15.04
  Codename:	vivid

  LVM version:     2.02.111(2) (2014-09-01)
  Library version: 1.02.90 (2014-09-01)
  Driver version:  4.28.0

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