Unable to create thinly provisioned snapshot of lvm volume

Issue

• We have created a non-thin lvm volume /dev/vgtest/lvtest, a separate thinpool and we are trying to create a thinly provisioned snapshot for it. But the lvcreate command fails to create a snapshot of /dev/vgtest/lvtest and shows following error messages:

  # lvs -a -o+devices
    LV                 VG     Attr       LSize    Pool Origin Data%  Move Log Cpy%Sync Convert Devices               
    lvtest             vgtest -wi-a-----  100.00m                                              /dev/mapper/mpatha(0)           <---- non-thin lvm volume
    mythinpool         vgtest twi-a-tz--  100.00m               0.00                           mythinpool_tdata(0)             <---- thin pool
    [mythinpool_tdata] vgtest Twi-ao----  100.00m                                              /dev/mapper/mpatha(26)
    [mythinpool_tmeta] vgtest ewi-ao----    4.00m                                              /dev/mapper/mpathd(0) 
  

  Tried creating thin snapshot, but it says /dev/vgtest/lvtest is writable:

  # lvcreate -s --thinpool vgtest/mythinpool lv1 --name mysnap
    Cannot use writable LV as the external origin.