title: How to tell Docker to use dm/LVM backend for volumes instead of vfs
body: <p>I recently heard (from a RedHat guy) that "direct-LVM"(devicemapper) is the recommended storage-backend for production setups, so I wanted to try that out on a CentOS 7 VM. (where loopback-LVM seems to be the default).</p>
  
  <p>So I created a separate data-disk and VG with 2 LVs for data and metadata, passed them into the docker config and started up docker ... so far so good, looks like this:</p>
  
  <pre><code># ps auxwf
  ...
  /usr/bin/docker -d --selinux-enabled -H unix://var/run/docker.sock \
                  --log-level=warn --storage-opt dm.fs=xfs \
                  --storage-opt dm.datadev=/dev/vg_data/docker-data \
                  --storage-opt dm.metadatadev=/dev/vg_data/docker-meta \
                  --storage-opt dm.basesize=30G --bip=172.17.42.1/24 \
  
  # docker info
  Containers: 8
  Images: 145
  Storage Driver: devicemapper
   Pool Name: docker-253:0-34485692-pool
   Pool Blocksize: 65.54 kB
   Backing Filesystem: xfs
   Data file: /dev/vg_data/docker-data
   Metadata file: /dev/vg_data/docker-meta
   Data Space Used: 4.498 GB
   Data Space Total: 34.36 GB
   Data Space Available: 29.86 GB
   Metadata Space Used: 6.402 MB
   Metadata Space Total: 104.9 MB
   Metadata Space Available: 98.46 MB
   ...
   ...
  </code></pre>
  
  <p>But today when I started a container that generates pretty much of local data (since I don't need to persist that for testing, I haven't mapped that Volumes anywhere on startup) , I noticed, that Volume-Data is all put into <code>/var/lib/docker/vfs</code> directories instead of the LVM thinpool as I expected.</p>
  
  <p>This is in fact filling up my root-fs that I kept small by intent.</p>
  
  <p>This is the Disk-Layout as seen by the Docker-Host:</p>
  
  <pre><code># lsblk
  NAME                             MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
  sda                                8:0    0   10G  0 disk 
  +-sda1                             8:1    0  500M  0 part /boot
  +-sda2                             8:2    0  9.5G  0 part 
    +-centos_system-root           253:0    0  8.5G  0 lvm  /
    +-centos_system-swap           253:1    0    1G  0 lvm  [SWAP]
  sdb                                8:16   0   50G  0 disk 
  +-vg_data-docker--data           253:2    0   32G  0 lvm  
  | +-docker-253:0-34485692-pool   253:4    0   32G  0 dm   
  |   +-docker-253:0-34485692-...  253:5    0   30G  0 dm   
  |   +-docker-253:0-34485692-...  253:6    0   30G  0 dm   
  +-vg_data-docker--meta           253:3    0  100M  0 lvm  
    +-docker-253:0-34485692-pool   253:4    0   32G  0 dm   
      +-docker-253:0-34485692-...  253:5    0   30G  0 dm   
      +-docker-253:0-34485692-...  253:6    0   30G  0 dm   
  </code></pre>
  
  <p>How can I get Docker to put Volumes (either implicitly created or explicit  in Data-containers) to be put onto the configured storage-backend ?</p>
  
  <p>Or will this really only be used for (base-)images and my expectations have been totally wrong ?</p>