Docker device-mapper Storage Backend

External

• https://docs.docker.com/storage/storagedriver/
• https://docs.docker.com/storage/storagedriver/device-mapper-driver/
• https://docs.docker.com/engine/reference/commandline/dockerd/#options-per-storage-driver

Internal

• Docker Concepts - Storage Backend

Overview

Device-mapper driver metadata is stored in /var/lib/docker/devicemapper/metadata. Docker state can be reset by stopping docker and by removing /var/lib/docker/devicemapper/metadata directory. This will destroy existing docker images and containers and all the docker metadata.

The thin pool.

Base device.

/var/lib/docker/devicemapper/metadata/base
{"device_id":1,"size":10737418240,"transaction_id":1,"initialized":true,"deleted":false}

What is metadata in this context? The data that is maintained in /var/lib/docker/devicemapper/metadata?

The concept of a snapshot of an image.

The concept of a devicemapper snapshot.

There seems to be a base device for each running container.

How are images and container stored.

A container in this context is a writable layer.

Layer.

Thin pool block and thin pool block size.

Info

At runtime, general statistics on the devicemapper storage driver can be obtained with:

docker info
Storage Driver: devicemapper
 Pool Name: docker_vg-container--thinpool
 Pool Blocksize: 524.3 kB
 Base Device Size: 10.74 GB
 Backing Filesystem: xfs
 Data file:
 Metadata file:
 Data Space Used: 16.81 GB
 Data Space Total: 53.57 GB
 Data Space Available: 36.76 GB
 Metadata Space Used: 3.338 MB
 Metadata Space Total: 54.53 MB
 Metadata Space Available: 51.19 MB
 Thin Pool Minimum Free Space: 5.357 GB
 Udev Sync Supported: true
 Deferred Removal Enabled: true
 Deferred Deletion Enabled: true
 Deferred Deleted Device Count: 0
 Library Version: 1.02.140-RHEL7 (2017-05-03)
 ...

Base Device Size

https://www.projectatomic.io/blog/2016/03/daemon_option_basedevicesize/

Base size is the maximum size that a image, and containers based on it, can grow to. Essentially, the size of the root filesystem of the container.

View Base Device Size

The docker engine-wide base image size is reported with docker info (see above). The container-specific base image size is reported with docker inspect {{.GraphDriver.Data.DeviceSize}}:

...
"GraphDriver": {
   "Name": "devicemapper",
   "Data": {
       "DeviceId": "3315",
       "DeviceName": "docker-253:3-12968682-7827556d545926857d669f5f9bb7fd69083e7e98636ff3f060d352223f4918ab",
       "DeviceSize": "10737418240"
   }
}
...

At runtime, the same amount can be viewed by attaching to the container and execute:

df -h
Filesystem                               Size  Used Avail Use% Mounted on
/dev/mapper/docker-253:3-12968682-....   10G  658M  9.4G   7% /

Modify Base Device Size

The docker engine instance wide value set with:

--storage-opt dm.basesize=20G

on docker engine's command line or configuration file.

For a specific container, it can be set with:

docker run --storage-opt size=60

Apparently it can only be expanded, not shrunk - needs experimentation. Existing images will retain the old base device size, unless they are rmi/pull.