How do I clear a thinpool device for docker

Question

I am running docker on a Redhat system with devicemapper and thinpool device just as recommended for production systems. Now when I want to reinstall docker I need two steps:

1) remove docker directory (in my case /area51/docker)
2) clear thinpool device

The docker documentation states that when using devicemapper with dm.metadev and dm.datadev options, the easiest way of cleaning devicemapper would be:

If setting up a new metadata pool it is required to be valid. This can be achieved by zeroing the first 4k to indicate empty metadata, like this:

$ dd if=/dev/zero of=$metadata_dev bs=4096 count=1

Unfortunately, according to the documentation, the dm.metadatadev is deprecated, it says to use dm.thinpooldev instead.

My thinpool has been created along the lines of this docker instruction So, my setup now looks like this:

cat /etc/docker/daemon.json  
{
        "storage-driver": "devicemapper", 
        "storage-opts": [ 
        "dm.thinpooldev=/dev/mapper/thinpool_VG_38401-thinpool",
        "dm.basesize=18G"
        ]  
}

Under the devicemapper directory i see the following thinpool devices

ls -l /dev/mapper/thinpool_VG_38401-thinpool*
lrwxrwxrwx 1 root root 7 Dec  6 08:31 /dev/mapper/thinpool_VG_38401-thinpool -> ../dm-8
lrwxrwxrwx 1 root root 7 Dec  6 08:31 /dev/mapper/thinpool_VG_38401-thinpool_tdata -> ../dm-7
lrwxrwxrwx 1 root root 7 Dec  6 08:31 /dev/mapper/thinpool_VG_38401-thinpool_tmeta -> ../dm-6

So, after running docker successfully I tried to reinstall as described above and clear the thinpool by writing 4K zeroes into the tmeta device and restart docker:

dd if=/dev/zero of=/dev/mapper/thinpool_VG_38401-thinpool_tmeta bs=4096 count=1  
systemctl start docker

And endet up with

docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Wed 2017-12-06 10:28:46 UTC; 10s ago
     Docs: https://docs.docker.com
  Process: 1566 ExecStart=/usr/bin/dockerd -G uwsgi --data-root=/area51/docker -H unix:///var/run/docker.sock (code=exited, status=1/FAILURE)
 Main PID: 1566 (code=exited, status=1/FAILURE)
   Memory: 236.0K
   CGroup: /system.slice/docker.service

Dec 06 10:28:45 yoda3 systemd[1]: Starting Docker Application Container Engine...
Dec 06 10:28:45 yoda3 dockerd[1566]: time="2017-12-06T10:28:45.816049000Z" level=info msg="libcontainerd: new containerd process, pid: 1577"
Dec 06 10:28:46 yoda3 dockerd[1566]: time="2017-12-06T10:28:46.816966000Z" level=warning msg="failed to rename /area51/docker/tmp for background deletion: renam...chronously"
Dec 06 10:28:46 yoda3 dockerd[1566]: Error starting daemon: error initializing graphdriver: devmapper: Unable to take ownership of thin-pool (thinpool_VG_38401-...data blocks
Dec 06 10:28:46 yoda3 systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
Dec 06 10:28:46 yoda3 systemd[1]: Failed to start Docker Application Container Engine.
Dec 06 10:28:46 yoda3 systemd[1]: Unit docker.service entered failed state.
Dec 06 10:28:46 yoda3 systemd[1]: docker.service failed.

I assumed I could get around the 'unable to take ownership of thin-pool' by doing a reboot. But after reboot and trying to start docker again I got the following error:

systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Wed 2017-12-06 10:30:37 UTC; 2min 29s ago
     Docs: https://docs.docker.com
  Process: 3180 ExecStart=/usr/bin/dockerd -G uwsgi --data-root=/area51/docker -H unix:///var/run/docker.sock (code=exited, status=1/FAILURE)
 Main PID: 3180 (code=exited, status=1/FAILURE)
   Memory: 37.9M
   CGroup: /system.slice/docker.service

Dec 06 10:30:36 yoda3 systemd[1]: Starting Docker Application Container Engine...
Dec 06 10:30:36 yoda3 dockerd[3180]: time="2017-12-06T10:30:36.893777000Z" level=warning msg="libcontainerd: makeUpgradeProof could not open /var/run/docker/lib...containerd"
Dec 06 10:30:36 yoda3 dockerd[3180]: time="2017-12-06T10:30:36.901958000Z" level=info msg="libcontainerd: new containerd process, pid: 3224"
Dec 06 10:30:37 yoda3 dockerd[3180]: Error starting daemon: error initializing graphdriver: devicemapper: Non existing device thinpool_VG_38401-thinpool 
Dec 06 10:30:37 yoda3 systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
Dec 06 10:30:37 yoda3 systemd[1]: Failed to start Docker Application Container Engine.
Dec 06 10:30:37 yoda3 systemd[1]: Unit docker.service entered failed state.
Dec 06 10:30:37 yoda3 systemd[1]: docker.service failed.

So, obviously writing zeroes into the thinpool_meta device is not the right thing to do, it seems to destroy my thinpool device.

Anyone here that can tell me the right steps to clear the thin-pool device? Preferably the solution should not require a reboot.