devmapper

README

devicemapper - a storage backend based on Device Mapper

Theory of operation

The device mapper graphdriver uses the device mapper thin provisioning module (dm-thinp) to implement CoW snapshots. For each devicemapper graph location (typically /var/lib/docker/devicemapper, $graph below) a thin pool is created based on two block devices, one for data and one for metadata. By default these block devices are created automatically by using loopback mounts of automatically created sparse files.

The default loopback files used are $graph/devicemapper/data and $graph/devicemapper/metadata. Additional metadata required to map from docker entities to the corresponding devicemapper volumes is stored in the $graph/devicemapper/json file (encoded as Json).

In order to support multiple devicemapper graphs on a system, the thin pool will be named something like: docker-0:33-19478248-pool, where the 0:33 part is the minor/major device nr and 19478248 is the inode number of the $graph directory.

On the thin pool, docker automatically creates a base thin device, called something like docker-0:33-19478248-base of a fixed size. This is automatically formatted with an empty filesystem on creation. This device is the base of all docker images and containers. All base images are snapshots of this device and those images are then in turn used as snapshots for other images and eventually containers.

Information on docker info

As of docker-1.4.1, docker info when using the devicemapper storage driver will display something like:

$ sudo docker info
[...]
Storage Driver: devicemapper
 Pool Name: docker-253:1-17538953-pool
 Pool Blocksize: 65.54 kB
 Data file: /dev/loop4
 Metadata file: /dev/loop4
 Data Space Used: 2.536 GB
 Data Space Total: 107.4 GB
 Data Space Available: 104.8 GB
 Metadata Space Used: 7.93 MB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.14 GB
 Udev Sync Supported: true
 Data loop file: /home/docker/devicemapper/devicemapper/data
 Metadata loop file: /home/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.82-git (2013-10-04)
[...]

status items

Each item in the indented section under Storage Driver: devicemapper are status information about the driver.

• Pool Name name of the devicemapper pool for this driver.
• Pool Blocksize tells the blocksize the thin pool was initialized with. This only changes on creation.
• Data file blockdevice file used for the devicemapper data
• Metadata file blockdevice file used for the devicemapper metadata
• Data Space Used tells how much of Data file is currently used
• Data Space Total tells max size the Data file
• Data Space Available tells how much free space there is in the Data file. If you are using a loop device this will report the actual space available to the loop device on the underlying filesystem.
• Metadata Space Used tells how much of Metadata file is currently used
• Metadata Space Total tells max size the Metadata file
• Metadata Space Available tells how much free space there is in the Metadata file. If you are using a loop device this will report the actual space available to the loop device on the underlying filesystem.
• Udev Sync Supported tells whether devicemapper is able to sync with Udev. Should be true.
• Data loop file file attached to Data file, if loopback device is used
• Metadata loop file file attached to Metadata file, if loopback device is used
• Library Version from the libdevmapper used

About the devicemapper options

The devicemapper backend supports some options that you can specify when starting the docker daemon using the --storage-opt flags. This uses the dm prefix and would be used something like docker -d --storage-opt dm.foo=bar.

These options are currently documented both in the man page and in [the online documentation](https://docs.docker.com/reference/commandline/daemon/#docker- execdriver-option). If you add an options, update both the man page and the documentation.

Documentation

Index

• func Init(home string, options []string) (graphdriver.Driver, error)
• func Mounted(mountpoint string) (bool, error)
• func ProbeFsType(device string) (string, error)
• type DevStatus
• type DeviceSet
  • func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error)
  • func (devices *DeviceSet) AddDevice(hash, baseHash string) error
  • func (devices *DeviceSet) DMLog(level int, file string, line int, dmError int, message string)
  • func (devices *DeviceSet) DataDevicePath() string
  • func (devices *DeviceSet) DeleteDevice(hash string) error
  • func (devices *DeviceSet) GetDeviceStatus(hash string) (*DevStatus, error)
  • func (devices *DeviceSet) HasActivatedDevice(hash string) bool
  • func (devices *DeviceSet) HasDevice(hash string) bool
  • func (devices *DeviceSet) List() []string
  • func (devices *DeviceSet) MetadataDevicePath() string
  • func (devices *DeviceSet) MountDevice(hash, path, mountLabel string) error
  • func (devices *DeviceSet) ResizePool(size int64) error
  • func (devices *DeviceSet) Shutdown() error
  • func (devices *DeviceSet) Status() *Status
  • func (devices *DeviceSet) UnmountDevice(hash string) error
• type DiskUsage
• type Driver
  • func (d *Driver) Cleanup() error
  • func (d *Driver) Create(id, parent string) error
  • func (d *Driver) Exists(id string) bool
  • func (d *Driver) Get(id, mountLabel string) (string, error)
  • func (d *Driver) GetMetadata(id string) (map[string]string, error)
  • func (d *Driver) Put(id string) error
  • func (d *Driver) Remove(id string) error
  • func (d *Driver) Status() [][2]string
  • func (d *Driver) String() string
• type Status

Functions

func Init

func Init(home string, options []string) (graphdriver.Driver, error)

Init creates a driver with the given home and the set of options.

func Mounted

func Mounted(mountpoint string) (bool, error)

Mounted returns true if a mount point exists.

func ProbeFsType

func ProbeFsType(device string) (string, error)

ProbeFsType returns the filesystem name for the given device id.

Types

type DevStatus

type DevStatus struct {
	// DeviceID is the id of the device.
	DeviceID int
	// Size is the size of the filesystem.
	Size uint64
	// TransactionID is a unique integer per device set used to identify an operation on the file system, this number is incremental.
	TransactionID uint64
	// SizeInSectors indicates the size of the sectors allocated.
	SizeInSectors uint64
	// MappedSectors indicates number of mapped sectors.
	MappedSectors uint64
	// HighestMappedSector is the pointer to the highest mapped sector.
	HighestMappedSector uint64
}

DevStatus returns information about device mounted containing its id, size and sector information.

type DeviceSet

type DeviceSet struct {
	sync.Mutex `json:"-"` // Protects Devices map and serializes calls into libdevmapper

	TransactionID uint64 `json:"-"`
	NextDeviceID  int    `json:"next_device_id"`

	BaseDeviceUUID string //save UUID of base device
	// contains filtered or unexported fields
}

DeviceSet holds information about list of devices

func NewDeviceSet

func NewDeviceSet(root string, doInit bool, options []string) (*DeviceSet, error)

NewDeviceSet creates the device set based on the options provided.

func (*DeviceSet) AddDevice

func (devices *DeviceSet) AddDevice(hash, baseHash string) error

AddDevice adds a device and registers in the hash.

func (*DeviceSet) DMLog

func (devices *DeviceSet) DMLog(level int, file string, line int, dmError int, message string)

DMLog implements logging using DevMapperLogger interface.

func (*DeviceSet) DataDevicePath

func (devices *DeviceSet) DataDevicePath() string

DataDevicePath returns the path to the data storage for this deviceset, regardless of loopback or block device

func (*DeviceSet) DeleteDevice

func (devices *DeviceSet) DeleteDevice(hash string) error

DeleteDevice deletes a device from the hash.

func (*DeviceSet) GetDeviceStatus

func (devices *DeviceSet) GetDeviceStatus(hash string) (*DevStatus, error)

GetDeviceStatus provides size, mapped sectors

func (*DeviceSet) HasActivatedDevice

func (devices *DeviceSet) HasActivatedDevice(hash string) bool

HasActivatedDevice return true if the device exists.

func (*DeviceSet) HasDevice

func (devices *DeviceSet) HasDevice(hash string) bool

HasDevice returns true if the device is in the hash and mounted.

func (*DeviceSet) List

func (devices *DeviceSet) List() []string

List returns a list of device ids.

func (*DeviceSet) MetadataDevicePath

func (devices *DeviceSet) MetadataDevicePath() string

MetadataDevicePath returns the path to the metadata storage for this deviceset, regardless of loopback or block device

func (*DeviceSet) MountDevice

func (devices *DeviceSet) MountDevice(hash, path, mountLabel string) error

MountDevice mounts the device if not already mounted.

func (*DeviceSet) ResizePool

func (devices *DeviceSet) ResizePool(size int64) error

ResizePool increases the size of the pool.

func (*DeviceSet) Shutdown

func (devices *DeviceSet) Shutdown() error

Shutdown shuts down the device by unmounting the root.

func (*DeviceSet) Status

func (devices *DeviceSet) Status() *Status

Status returns the current status of this deviceset

func (*DeviceSet) UnmountDevice

func (devices *DeviceSet) UnmountDevice(hash string) error

UnmountDevice unmounts the device and removes it from hash.

type DiskUsage

type DiskUsage struct {
	// Used bytes on the disk.
	Used uint64
	// Total bytes on the disk.
	Total uint64
	// Available bytes on the disk.
	Available uint64
}

DiskUsage contains information about disk usage and is used when reporting Status of a device.

type Driver

type Driver struct {
	*DeviceSet
	// contains filtered or unexported fields
}

Driver contains the device set mounted and the home directory

func (*Driver) Cleanup

func (d *Driver) Cleanup() error

Cleanup unmounts a device.

func (*Driver) Create

func (d *Driver) Create(id, parent string) error

Create adds a device with a given id and the parent.

func (*Driver) Exists

func (d *Driver) Exists(id string) bool

Exists checks to see if the device is mounted.

func (*Driver) Get

func (d *Driver) Get(id, mountLabel string) (string, error)

Get mounts a device with given id into the root filesystem

func (*Driver) GetMetadata

func (d *Driver) GetMetadata(id string) (map[string]string, error)

GetMetadata returns a map of information about the device.

func (*Driver) Put

func (d *Driver) Put(id string) error

Put unmounts a device and removes it.

func (*Driver) Remove

func (d *Driver) Remove(id string) error

Remove removes a device with a given id, unmounts the filesystem.

func (*Driver) Status

func (d *Driver) Status() [][2]string

Status returns the status about the driver in a printable format. Information returned contains Pool Name, Data File, Metadata file, disk usage by the data and metadata, etc.

func (*Driver) String

func (d *Driver) String() string

type Status

type Status struct {
	// PoolName is the name of the data pool.
	PoolName string
	// DataFile is the actual block device for data.
	DataFile string
	// DataLoopback loopback file, if used.
	DataLoopback string
	// MetadataFile is the actual block device for metadata.
	MetadataFile string
	// MetadataLoopback is the loopback file, if used.
	MetadataLoopback string
	// Data is the disk used for data.
	Data DiskUsage
	// Metadata is the disk used for meta data.
	Metadata DiskUsage
	// SectorSize size of the vector.
	SectorSize uint64
	// UdevSyncSupported is true if sync is supported.
	UdevSyncSupported bool
	// DeferredRemoveEnabled is true then the device is not unmounted.
	DeferredRemoveEnabled bool
}

Status returns the information about the device.