devmapper: Add option for specifying an lvm2 created thin-pool device

Ideally lvm2 would be used to create/manage the thin-pool volume that is
then handed to docker to exclusively create/manage the thin and thin
snapshot volumes needed for it's containers.  Managing the thin-pool
outside of docker makes for the most feature-rich method of having
docker utilize device mapper thin provisioning as the backing storage
for docker's containers.  lvm2-based thin-pool management feature
highlights include: automatic or interactive thin-pool resize support,
dynamically change thin-pool features, automatic thinp metadata checking
when lvm2 activates the thin-pool, etc.

Docker will not activate/deactivate the specified thin-pool device but
it will exclusively manage/create thin and thin snapshot volumes in it.

Docker will not take ownership of the specified thin-pool device unless
it has 0 data blocks used and a transaction id of 0.  This should help
guard against using a thin-pool that is already in use.

Also fix typos in setupBaseImage() relative to the thin volume type of
the base image.

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)

daemon/graphdriver/devmapper/README.md

@@ -100,6 +100,25 @@ Here is the list of supported options:
 
     ``docker -d --storage-opt dm.mountopt=nodiscard``
 
+ *  `dm.thinpooldev`
+
+    Specifies a custom blockdevice to use for the thin pool.
+
+    If using a block device for device mapper storage, ideally lvm2
+    would be used to create/manage the thin-pool volume that is then
+    handed to docker to exclusively create/manage the thin and thin
+    snapshot volumes needed for it's containers.  Managing the thin-pool
+    outside of docker makes for the most feature-rich method of having
+    docker utilize device mapper thin provisioning as the backing
+    storage for docker's containers.  lvm2-based thin-pool management
+    feature highlights include: automatic or interactive thin-pool
+    resize support, dynamically change thin-pool features, automatic
+    thinp metadata checking when lvm2 activates the thin-pool, etc.
+
+    Example use:
+
+    ``docker -d --storage-opt dm.thinpooldev=/dev/mapper/thin-pool``
+
  *  `dm.datadev`
 
     Specifies a custom blockdevice to use for data for the thin pool.

daemon/graphdriver/devmapper/deviceset.go

@@ -84,6 +84,7 @@ type DeviceSet struct {
 	metadataDevice       string
 	doBlkDiscard         bool
 	thinpBlockSize       uint32
+	thinPoolDevice       string
 }
 
 type DiskUsage struct {
@@ -150,7 +151,11 @@ func (devices *DeviceSet) oldMetadataFile() string {
 }
 
 func (devices *DeviceSet) getPoolName() string {
-	return devices.devicePrefix + "-pool"
+	if devices.thinPoolDevice == "" {
+		return devices.devicePrefix + "-pool"
+	} else {
+		return devices.thinPoolDevice
+	}
 }
 
 func (devices *DeviceSet) getPoolDevName() string {
@@ -411,7 +416,21 @@ func (devices *DeviceSet) setupBaseImage() error {
 		}
 	}
 
-	log.Debugf("Initializing base device-manager snapshot")
+	if devices.thinPoolDevice != "" && oldInfo == nil {
+		if _, transactionId, dataUsed, _, _, _, err := devices.poolStatus(); err != nil {
+			return err
+		} else {
+			if dataUsed != 0 {
+				return fmt.Errorf("Unable to take ownership of thin-pool (%s) that already has used data blocks",
+					devices.thinPoolDevice)
+			} else if transactionId != 0 {
+				return fmt.Errorf("Unable to take ownership of thin-pool (%s) with non-zero transaction Id",
+					devices.thinPoolDevice)
+			}
+		}
+	}
+
+	log.Debugf("Initializing base device-mapper thin volume")
 
 	id := devices.NextDeviceId
 
@@ -430,7 +449,7 @@ func (devices *DeviceSet) setupBaseImage() error {
 		return err
 	}
 
-	log.Debugf("Creating filesystem on base device-manager snapshot")
+	log.Debugf("Creating filesystem on base device-mapper thin volume")
 
 	if err = devices.activateDeviceIfNeeded(info); err != nil {
 		return err
@@ -605,7 +624,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 	devices.devicePrefix = fmt.Sprintf("docker-%d:%d-%d", major(sysSt.Dev), minor(sysSt.Dev), sysSt.Ino)
 	log.Debugf("Generated prefix: %s", devices.devicePrefix)
 
-	// Check for the existence of the device <prefix>-pool
+	// Check for the existence of the thin-pool device
 	log.Debugf("Checking for existence of the pool '%s'", devices.getPoolName())
 	info, err := devicemapper.GetInfo(devices.getPoolName())
 	if info == nil {
@@ -624,7 +643,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 	createdLoopback := false
 
 	// If the pool doesn't exist, create it
-	if info.Exists == 0 {
+	if info.Exists == 0 && devices.thinPoolDevice == "" {
 		log.Debugf("Pool doesn't exist. Creating it.")
 
 		var (
@@ -988,8 +1007,10 @@ func (devices *DeviceSet) Shutdown() error {
 	}
 
 	devices.Lock()
-	if err := devices.deactivatePool(); err != nil {
-		log.Debugf("Shutdown deactivate pool , error: %s", err)
+	if devices.thinPoolDevice == "" {
+		if err := devices.deactivatePool(); err != nil {
+			log.Debugf("Shutdown deactivate pool , error: %s", err)
+		}
 	}
 
 	devices.saveDeviceSetMetaData()
@@ -1275,6 +1296,8 @@ func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error
 			devices.metadataDevice = val
 		case "dm.datadev":
 			devices.dataDevice = val
+		case "dm.thinpooldev":
+			devices.thinPoolDevice = strings.TrimPrefix(val, "/dev/mapper/")
 		case "dm.blkdiscard":
 			foundBlkDiscard = true
 			devices.doBlkDiscard, err = strconv.ParseBool(val)