Enabling direct-lvm mode for devicemapper storage drivers.

This document (7018751) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 12 Service Pack 2 (SLES 12 SP2)
SUSE Linux Enterprise Server 12 Service Pack 1 (SLES 12 SP1)

Situation

The following error is generated when Docker is using "devicemapper" storage drivers with default configuration known as "loop-lvm" :

WARNING: Usage of loopback devices is strongly discouraged for production use.
Use `--storage-opt dm.thinpooldev` to specify a custom block storage device.

Resolution

The "direct-lvm" mode uses block devices for creating the thin pool instead of loopback devices, to remove the WARNING message and set up the container to use a block device, perform the following steps :

(note: this procedure assumes that a spare block device is available, than substitute /dev/sdX):

Stop Docker:

systemctl stop docker.service

Install LVM2 package, when not installed already:

zypper in lvm2

Create a physical volume :

pvcreate /dev/sdX

Create a "docker" volume group :

vgcreate docker /dev/sdX

Create a logical volume named thinpool and thinpoolmeta :

lvcreate --wipesignatures y -n thinpool docker -l 95%VG && lvcreate --wipesignatures y -n thinpoolmeta docker -l 1%VG

NOTE: The data logical volume is adviced to be 95% of the volume group size to leave free space for auto expanding the data or metadata (in case the disk space runs low).

Convert the pool to a thin pool :

lvconvert -y --zero n -c 512K --thinpool docker/thinpool --poolmetadata docker/thinpoolmeta

Configure auto extension of thin pools via an lvm profile:

vi /etc/lvm/profile/docker-thinpool.profile

Insert the following lines:

activation {

thin_pool_autoextend_threshold=80
thin_pool_autoextend_percent=20
}

Apply the lvm profile :

lvchange --metadataprofile docker-thinpool docker/thinpool

Move the graph driver:

mkdir /var/lib/docker.bk && mv /var/lib/docker/* /var/lib/docker.bk

(Important note : Moving the graph driver also remove images, containers and volumes !!!)

Configure the Docker option to use the newly created block device:

vi /etc/sysconfig/docker

Insert the following:

DOCKER_OPTS="-s devicemapper --storage-opt=dm.thinpooldev=/dev/mapper/docker-thinpool"
NOTE: Alternatively, you can specify the option "--storage-opt" when launching containers with "docker run".

Reload systemd for applying the changes :

systemctl daemon-reload

Start Docker:

systemctl start docker.service

Verify that the WARNING has disappeared :

docker info

Cause

With "loop-lvm" mode, Docker is using a loopback device to build the thin pool used by image and container snapshots, this configuration is the default one for "devicemapper" storage drivers and it is strongly discouraged in a production environment.

Additional Information

It is recommended to push the images to a Docker Registry such as Portus before setting up the "direct-lvm" mode.

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.