@@ -186,7 +186,7 @@ Here is the list of supported options:

     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```

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

     Example use:

@@ -216,3 +216,39 @@ Here is the list of supported options:

     Example use:

     ``docker -d --storage-opt dm.blkdiscard=false``

+

+ *  `dm.override_udev_sync_check`

+

+    Overrides the `udev` synchronization checks between `devicemapper` and `udev`.

+    `udev` is the device manager for the Linux kernel.

+

+    To view the `udev` sync support of a Docker daemon that is using the

+    `devicemapper` driver, run:

+

+        $ docker info

+	[...]

+	 Udev Sync Supported: true

+	[...]

+

+    When `udev` sync support is `true`, then `devicemapper` and udev can

+    coordinate the activation and deactivation of devices for containers.

+

+    When `udev` sync support is `false`, a race condition occurs between

+    the`devicemapper` and `udev` during create and cleanup. The race condition

+    results in errors and failures. (For information on these failures, see

+    [docker#4036](https://github.com/docker/docker/issues/4036))

+

+    To allow the `docker` daemon to start, regardless of `udev` sync not being

+    supported, set `dm.override_udev_sync_check` to true:

+

+        $ docker -d --storage-opt dm.override_udev_sync_check=true

+

+    When this value is `true`, the  `devicemapper` continues and simply warns

+    you the errors are happening.

+

+    > **Note**: The ideal is to pursue a `docker` daemon and environment that

+    > does support synchronizing with `udev`. For further discussion on this

+    > topic, see [docker#4036](https://github.com/docker/docker/issues/4036).

+    > Otherwise, set this flag for migrating existing Docker daemons to a

+    > daemon with a supported environment.

+

@@ -30,7 +30,8 @@ var (

 	DefaultDataLoopbackSize     int64  = 100 * 1024 * 1024 * 1024

 	DefaultMetaDataLoopbackSize int64  = 2 * 1024 * 1024 * 1024

 	DefaultBaseFsSize           uint64 = 10 * 1024 * 1024 * 1024

-	DefaultThinpBlockSize       uint32 = 128      // 64K = 128 512b sectors

+	DefaultThinpBlockSize       uint32 = 128 // 64K = 128 512b sectors

+	DefaultUdevSyncOverride     bool   = false

 	MaxDeviceId                 int    = 0xffffff // 24 bit, pool limit

 	DeviceIdMapSz               int    = (MaxDeviceId + 1) / 8

 )

@@ -83,20 +84,21 @@ type DeviceSet struct {

 	deviceIdMap   []byte

 	// Options

-	dataLoopbackSize     int64

-	metaDataLoopbackSize int64

-	baseFsSize           uint64

-	filesystem           string

-	mountOptions         string

-	mkfsArgs             []string

-	dataDevice           string // block or loop dev

-	dataLoopFile         string // loopback file, if used

-	metadataDevice       string // block or loop dev

-	metadataLoopFile     string // loopback file, if used

-	doBlkDiscard         bool

-	thinpBlockSize       uint32

-	thinPoolDevice       string

-	Transaction          `json:"-"`

+	dataLoopbackSize      int64

+	metaDataLoopbackSize  int64

+	baseFsSize            uint64

+	filesystem            string

+	mountOptions          string

+	mkfsArgs              []string

+	dataDevice            string // block or loop dev

+	dataLoopFile          string // loopback file, if used

+	metadataDevice        string // block or loop dev

+	metadataLoopFile      string // loopback file, if used

+	doBlkDiscard          bool

+	thinpBlockSize        uint32

+	thinPoolDevice        string

+	Transaction           `json:"-"`

+	overrideUdevSyncCheck bool

 }

 type DiskUsage struct {

@@ -963,8 +965,10 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {

 	// https://github.com/docker/docker/issues/4036

 	if supported := devicemapper.UdevSetSyncSupport(true); !supported {

-		logrus.Errorf("Udev sync is not supported. This will lead to unexpected behavior, data loss and errors")

-		return graphdriver.ErrNotSupported

+		logrus.Errorf("Udev sync is not supported. This will lead to unexpected behavior, data loss and errors. For more information, see https://docs.docker.com/reference/commandline/cli/#daemon-storage-driver-option")

+		if !devices.overrideUdevSyncCheck {

+			return graphdriver.ErrNotSupported

+		}

 	}

 	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil && !os.IsExist(err) {

@@ -1656,15 +1660,16 @@ func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error

 	devicemapper.SetDevDir("/dev")

 	devices := &DeviceSet{

-		root:                 root,

-		MetaData:             MetaData{Devices: make(map[string]*DevInfo)},

-		dataLoopbackSize:     DefaultDataLoopbackSize,

-		metaDataLoopbackSize: DefaultMetaDataLoopbackSize,

-		baseFsSize:           DefaultBaseFsSize,

-		filesystem:           "ext4",

-		doBlkDiscard:         true,

-		thinpBlockSize:       DefaultThinpBlockSize,

-		deviceIdMap:          make([]byte, DeviceIdMapSz),

+		root:                  root,

+		MetaData:              MetaData{Devices: make(map[string]*DevInfo)},

+		dataLoopbackSize:      DefaultDataLoopbackSize,

+		metaDataLoopbackSize:  DefaultMetaDataLoopbackSize,

+		baseFsSize:            DefaultBaseFsSize,

+		overrideUdevSyncCheck: DefaultUdevSyncOverride,

+		filesystem:            "ext4",

+		doBlkDiscard:          true,

+		thinpBlockSize:        DefaultThinpBlockSize,

+		deviceIdMap:           make([]byte, DeviceIdMapSz),

 	}

 	foundBlkDiscard := false

@@ -1721,6 +1726,11 @@ func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error

 			}

 			// convert to 512b sectors

 			devices.thinpBlockSize = uint32(size) >> 9

+		case "dm.override_udev_sync_check":

+			devices.overrideUdevSyncCheck, err = strconv.ParseBool(val)

+			if err != nil {

+				return nil, err

+			}

 		default:

 			return nil, fmt.Errorf("Unknown option %s\n", key)

 		}

@@ -13,6 +13,7 @@ func init() {

 	DefaultDataLoopbackSize = 300 * 1024 * 1024

 	DefaultMetaDataLoopbackSize = 200 * 1024 * 1024

 	DefaultBaseFsSize = 300 * 1024 * 1024

+	DefaultUdevSyncOverride = true

 	if err := graphtest.InitLoopbacks(); err != nil {

 		panic(err)

 	}

@@ -370,6 +370,41 @@ Currently supported options are:

         $ docker -d --storage-opt dm.blkdiscard=false

+ *  `dm.override_udev_sync_check`

+

+    Overrides the `udev` synchronization checks between `devicemapper` and `udev`.

+    `udev` is the device manager for the Linux kernel.

+

+    To view the `udev` sync support of a Docker daemon that is using the

+    `devicemapper` driver, run:

+

+        $ docker info

+	[...]

+	 Udev Sync Supported: true

+	[...]

+

+    When `udev` sync support is `true`, then `devicemapper` and udev can

+    coordinate the activation and deactivation of devices for containers.

+

+    When `udev` sync support is `false`, a race condition occurs between

+    the`devicemapper` and `udev` during create and cleanup. The race condition

+    results in errors and failures. (For information on these failures, see

+    [docker#4036](https://github.com/docker/docker/issues/4036))

+

+    To allow the `docker` daemon to start, regardless of `udev` sync not being

+    supported, set `dm.override_udev_sync_check` to true:

+

+        $ docker -d --storage-opt dm.override_udev_sync_check=true

+

+    When this value is `true`, the  `devicemapper` continues and simply warns

+    you the errors are happening.

+

+    > **Note**: The ideal is to pursue a `docker` daemon and environment that

+    > does support synchronizing with `udev`. For further discussion on this

+    > topic, see [docker#4036](https://github.com/docker/docker/issues/4036).

+    > Otherwise, set this flag for migrating existing Docker daemons to a

+    > daemon with a supported environment.

+

 ### Docker exec-driver option

 The Docker daemon uses a specifically built `libcontainer` execution driver as its