George Moutsopoulos (gmoutso) wrote on 2015-04-29: 
> 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/module
Not exactly related to the original bug, but wanted to address a boo-boo in the 
sudo echo "string" won't use root's access/permissions to write to the files 
when using the shell-redirection operators '>' or '>>'.  sudo will execute 
'echo' with root privs, and that will echo "dm_cache" (et al.), however, that 
output is redirected by the *SHELL* (running as
"whoever is running the shell") -- usually the user.

To get around this problem (and not saying there might not be any easier ways, 
as this seemed a bit roundabout to get what I wanted (have root write the 
output to "wherever"), I used
"dd", as in:

echo "dm_cache"|sudo dd of=destination status=none

 or to append:

echo "dm_cache"|sudo dd status=none oflag=append conv=notrunc of=/etc

 or to do all at once:

(echo dm_cache
echo dm_cache_mq
echo dm_persistent_data
echo dm_bufio)|sudo dd status=none oflag=append conv=notrunc 

I wanted to write a single "char" to a file in /proc/sys/vm

Perhaps needless to say, I put it in a script file to save on typing.

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Touch seeded packages, which is subscribed to lvm2 in Ubuntu.

  Unable to mount lvmcache root device at boot time

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

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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