devmapper

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 Go to latest Published: Jun 17, 2015 License: Apache-2.0 Imports: 22 Imported by: 0

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

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.

Here is the list of supported options:

  • dm.basesize

    Specifies the size to use when creating the base device, which limits the size of images and containers. The default value is 10G. Note, thin devices are inherently "sparse", so a 10G device which is mostly empty doesn't use 10 GB of space on the pool. However, the filesystem will use more space for the empty case the larger the device is. Warning: This value affects the system-wide "base" empty filesystem that may already be initialized and inherited by pulled images. Typically, a change to this value will require additional steps to take effect: 1) stop docker -d, 2) rm -rf /var/lib/docker, 3) start docker -d.

    Example use:

    docker -d --storage-opt dm.basesize=20G

  • dm.loopdatasize

    Specifies the size to use when creating the loopback file for the "data" device which is used for the thin pool. The default size is 100G. Note that the file is sparse, so it will not initially take up this much space.

    Example use:

    docker -d --storage-opt dm.loopdatasize=200G

  • dm.loopmetadatasize

    Specifies the size to use when creating the loopback file for the "metadadata" device which is used for the thin pool. The default size is 2G. Note that the file is sparse, so it will not initially take up this much space.

    Example use:

    docker -d --storage-opt dm.loopmetadatasize=4G

  • dm.fs

    Specifies the filesystem type to use for the base device. The supported options are "ext4" and "xfs". The default is "ext4"

    Example use:

    docker -d --storage-opt dm.fs=xfs

  • dm.mkfsarg

    Specifies extra mkfs arguments to be used when creating the base device.

    Example use:

    docker -d --storage-opt "dm.mkfsarg=-O ^has_journal"

  • dm.mountopt

    Specifies extra mount options used when mounting the thin devices.

    Example use:

    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 its 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.

    If using a block device for device mapper storage, ideally both datadev and metadatadev should be specified to completely avoid using the loopback device.

    Example use:

    docker -d --storage-opt dm.datadev=/dev/sdb1 --storage-opt dm.metadatadev=/dev/sdc1

  • dm.metadatadev

    Specifies a custom blockdevice to use for metadata for the thin pool.

    For best performance the metadata should be on a different spindle than the data, or even better on an SSD.

    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

    Example use:

    docker -d --storage-opt dm.datadev=/dev/sdb1 --storage-opt dm.metadatadev=/dev/sdc1

  • dm.blocksize

    Specifies a custom blocksize to use for the thin pool. The default blocksize is 64K.

    Example use:

    docker -d --storage-opt dm.blocksize=512K

  • dm.blkdiscard

    Enables or disables the use of blkdiscard when removing devicemapper devices. This is enabled by default (only) if using loopback devices and is required to resparsify the loopback file on image/container removal.

    Disabling this on loopback can lead to much faster container removal times, but will make the space used in /var/lib/docker directory not be returned to the system for other use when containers are removed.

    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 thedevicemapper and udev during create and cleanup. The race condition results in errors and failures. (For information on these failures, see docker#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. Otherwise, set this flag for migrating existing Docker daemons to a daemon with a supported environment.

  • dm.use_deferred_removal

    Enables use of deferred device removal if libdm and kernel driver support the mechanism.

    Deferred device removal means that if device is busy when devices is being removed/deactivated, then a deferred removal is scheduled on device. And devices automatically goes away when last user of device exits.

    For example, when container exits, its associated thin device is removed. If that devices has leaked into some other mount namespace can can't be removed now, container exit will still be successful and this option will just schedule device for deferred removal and will not wait in a loop trying to remove a busy device.

    Example use:

    docker -d --storage-opt dm.use_deferred_removal=true

Documentation

Index

Constants

This section is empty.

Variables

View Source 
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
	DefaultUdevSyncOverride     bool   = false
	MaxDeviceId                 int    = 0xffffff // 24 bit, pool limit
	DeviceIdMapSz               int    = (MaxDeviceId + 1) / 8
	// We retry device removal so many a times that even error messages
	// will fill up console during normal operation. So only log Fatal
	// messages by default.
	DMLogLevel                   int  = devicemapper.LogLevelFatal
	DriverDeferredRemovalSupport bool = false
	EnableDeferredRemoval        bool = false
)

Functions

func Init 

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

func Mounted 

func Mounted(mountpoint string) (bool, error)

func ProbeFsType 

func ProbeFsType(device string) (string, error)

Types

type DevInfo 

type DevInfo struct {
	Hash          string `json:"-"`
	DeviceId      int    `json:"device_id"`
	Size          uint64 `json:"size"`
	TransactionId uint64 `json:"transaction_id"`
	Initialized   bool   `json:"initialized"`
	// contains filtered or unexported fields
}

func (*DevInfo) DevName 

func (info *DevInfo) DevName() string

func (*DevInfo) Name 

func (info *DevInfo) Name() string

type DevStatus 

type DevStatus struct {
	DeviceId            int
	Size                uint64
	TransactionId       uint64
	SizeInSectors       uint64
	MappedSectors       uint64
	HighestMappedSector uint64
}

type DeviceMetadata 

type DeviceMetadata struct {
	// contains filtered or unexported fields
}

Structure used to export image/container metadata in docker inspect.

type DeviceSet 

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

	TransactionId uint64 `json:"-"`
	NextDeviceId  int    `json:"next_device_id"`

	Transaction `json:"-"`
	// contains filtered or unexported fields
}

func NewDeviceSet 

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

func (*DeviceSet) AddDevice 

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

func (*DeviceSet) DMLog 

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

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

func (*DeviceSet) ExportDeviceMetadata 

func (devices *DeviceSet) ExportDeviceMetadata(hash string) (*DeviceMetadata, error)

Status returns the current status of this deviceset

func (*DeviceSet) GetDeviceStatus 

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

func (*DeviceSet) HasActivatedDevice 

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

func (*DeviceSet) HasDevice 

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

func (*DeviceSet) List 

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

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

func (*DeviceSet) ResizePool 

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

func (*DeviceSet) Shutdown 

func (devices *DeviceSet) Shutdown() error

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

type DiskUsage 

type DiskUsage struct {
	Used      uint64
	Total     uint64
	Available uint64
}

type Driver 

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

func (*Driver) Cleanup 

func (d *Driver) Cleanup() error

func (*Driver) Create 

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

func (*Driver) Exists 

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

func (*Driver) Get 

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

func (*Driver) GetMetadata 

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

func (*Driver) Put 

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

func (*Driver) Remove 

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

func (*Driver) Status 

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

func (*Driver) String 

func (d *Driver) String() string

type MetaData 

type MetaData struct {
	Devices map[string]*DevInfo `json:"Devices"`
	// contains filtered or unexported fields
}

type Status 

type Status struct {
	PoolName              string
	DataFile              string // actual block device for data
	DataLoopback          string // loopback file, if used
	MetadataFile          string // actual block device for metadata
	MetadataLoopback      string // loopback file, if used
	Data                  DiskUsage
	Metadata              DiskUsage
	SectorSize            uint64
	UdevSyncSupported     bool
	DeferredRemoveEnabled bool
}

type Transaction 

type Transaction struct {
	OpenTransactionId uint64 `json:"open_transaction_id"`
	DeviceIdHash      string `json:"device_hash"`
	DeviceId          int    `json:"device_id"`
}

Source Files

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