containerd/snapshots/devmapper

Devmapper snapshotter

Devmapper is a containerd snapshotter plugin that stores snapshots in ext4-formatted filesystem images in a devicemapper thin pool.

Setup

To make it work you need to prepare thin-pool in advance and update containerd's configuration file. This file is typically located at /etc/containerd/config.toml.

Here's minimal sample entry that can be made in the configuration file:

[plugins]
  ...
  [plugins.devmapper]
    pool_name = "containerd-pool"
    base_image_size = "8192MB"
  ...

The following configuration flags are supported:

• root_path - a directory where the metadata will be available (if empty default location for containerd plugins will be used)
• pool_name - a name to use for the devicemapper thin pool. Pool name should be the same as in /dev/mapper/ directory
• base_image_size - defines how much space to allocate when creating the base device
• async_remove - flag to async remove device using snapshot GC's cleanup callback

Pool name and base image size are required snapshotter parameters.

Run

Give it a try with the following commands:

ctr images pull --snapshotter devmapper docker.io/library/hello-world:latest
ctr run --snapshotter devmapper docker.io/library/hello-world:latest test

Requirements

The devicemapper snapshotter requires dmsetup (>= 1.02.110) command line tool to be installed and available on your computer. On Ubuntu, it can be installed with apt-get install dmsetup command.

How to setup device mapper thin-pool

There are many ways how to configure a devmapper thin-pool depending on your requirements, disk configuration, and environment.

On local dev environment you can utilize loopback devices. This type of configuration is simple and suits well for development and testing (please note that this configuration is slow and not recommended for production uses). Run the following script to create a thin-pool device:

#!/bin/bash
set -ex

DATA_DIR=/var/lib/containerd/devmapper
POOL_NAME=devpool

mkdir -p ${DATA_DIR}

# Create data file
sudo touch "${DATA_DIR}/data"
sudo truncate -s 100G "${DATA_DIR}/data"

# Create metadata file
sudo touch "${DATA_DIR}/meta"
sudo truncate -s 10G "${DATA_DIR}/meta"

# Allocate loop devices
DATA_DEV=$(sudo losetup --find --show "${DATA_DIR}/data")
META_DEV=$(sudo losetup --find --show "${DATA_DIR}/meta")

# Define thin-pool parameters.
# See https://www.kernel.org/doc/Documentation/device-mapper/thin-provisioning.txt for details.
SECTOR_SIZE=512
DATA_SIZE="$(sudo blockdev --getsize64 -q ${DATA_DEV})"
LENGTH_IN_SECTORS=$(bc <<< "${DATA_SIZE}/${SECTOR_SIZE}")
DATA_BLOCK_SIZE=128
LOW_WATER_MARK=32768

# Create a thin-pool device
sudo dmsetup create "${POOL_NAME}" \
    --table "0 ${LENGTH_IN_SECTORS} thin-pool ${META_DEV} ${DATA_DEV} ${DATA_BLOCK_SIZE} ${LOW_WATER_MARK}"

cat << EOF
#
# Add this to your config.toml configuration file and restart containerd daemon
#
[plugins]
  [plugins.devmapper]
    pool_name = "${POOL_NAME}"
    root_path = "${DATA_DIR}"
    base_image_size = "10GB"
EOF

Use dmsetup to verify that the thin-pool created successfully:

sudo dmsetup ls
devpool	(253:0)

Once configured and restarted containerd, you'll see the following output:

INFO[2020-03-17T20:24:45.532604888Z] loading plugin "io.containerd.snapshotter.v1.devmapper"...  type=io.containerd.snapshotter.v1
INFO[2020-03-17T20:24:45.532672738Z] initializing pool device "dev-pool"

Another way to setup a thin-pool is via container-storage-setup tool (formerly known as docker-storage-setup). It is a script to configure CoW file systems like devicemapper:

#!/bin/bash
set -ex

# Block device to use for devmapper thin-pool
BLOCK_DEV=/dev/sdf
POOL_NAME=devpool
VG_NAME=containerd

# Install container-storage-setup tool
git clone https://github.com/projectatomic/container-storage-setup.git
cd container-storage-setup/
sudo make install-core
echo "Using version $(container-storage-setup -v)"

# Create configuration file
# Refer to `man container-storage-setup` to see available options
sudo tee /etc/sysconfig/docker-storage-setup <<EOF
DEVS=${BLOCK_DEV}
VG=${VG_NAME}
CONTAINER_THINPOOL=${POOL_NAME}
EOF

# Run the script
sudo container-storage-setup

cat << EOF
#
# Add this to your config.toml configuration file and restart containerd daemon
#
[plugins]
  [plugins.devmapper]
    pool_name = "${VG_NAME}-${POOL_NAME}"
    base_image_size = "10GB"
EOF

If successful container-storage-setup will output:

+ echo VG=containerd
+ sudo container-storage-setup
INFO: Volume group backing root filesystem could not be determined
INFO: Writing zeros to first 4MB of device /dev/xvdf
4+0 records in
4+0 records out
4194304 bytes (4.2 MB) copied, 0.0162906 s, 257 MB/s
INFO: Device node /dev/xvdf1 exists.
  Physical volume "/dev/xvdf1" successfully created.
  Volume group "containerd" successfully created
  Rounding up size to full physical extent 12.00 MiB
  Thin pool volume with chunk size 512.00 KiB can address at most 126.50 TiB of data.
  Logical volume "devpool" created.
  Logical volume containerd/devpool changed.
...

And dmsetup will produce the following output:

sudo dmsetup ls
containerd-devpool          (253:2)
containerd-devpool_tdata    (253:1)
containerd-devpool_tmeta    (253:0)