Devmapper is a containerd snapshotter plugin that stores snapshots in filesystem images in a Device-mapper thin-pool. The devmapper plugin takes advantage of Device-mapper features like device snapshot support.
To use the devmapper snapshotter plugin you need to prepare a Device-mapper thin-pool in advance and update containerd's configuration file. This file is typically located at /etc/containerd/config.toml.
Here's a minimal sample entry that can be made in the configuration file:
version = 2 [plugins] ... [plugins."io.containerd.snapshotter.v1.devmapper"] root_path = "/var/lib/containerd/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 Device-mapper thin-pool. Pool name should be the same as in /dev/mapper/ directorybase_image_size - defines how much space to allocate when creating thin device snapshots from the base (pool) deviceasync_remove - flag to async remove device using snapshot GC's cleanup callback (default: false)discard_blocks - whether to discard blocks when removing a device. This is especially useful for returning disk space to the filesystem when using loopback devices. (default: false)fs_type - defines the file system to use for snapshot device mount. Valid values are ext4 and xfs (default: "ext4")fs_options - optionally defines the file system options. This is currently only applicable to ext4 file system. (default: "")root_path, pool_name, and base_image_size are required snapshotter parameters.
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
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.
There are many ways how to configure a Device-mapper thin-pool depending on your requirements, disk configuration, and environment. Two common configurations are provided below, one for development environments and one for production environments.
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 with associated metadata and data device files:
#!/bin/bash set -ex DATA_DIR=/var/lib/containerd/devmapper POOL_NAME=devpool sudo 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} ${LOW_WATER_MARK}" cat << EOF # # Add this to your config.toml configuration file and restart the containerd daemon # [plugins] [plugins."io.containerd.snapshotter.v1.devmapper"] pool_name = "${POOL_NAME}" root_path = "${DATA_DIR}" base_image_size = "10GB" discard_blocks = true EOF
Use dmsetup to verify that the thin-pool was created successfully:
sudo dmsetup ls devpool (253:0)
Once containerd is configured and restarted, 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 "devpool"
Another way to setup a thin-pool is via the 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] root_path = "/var/lib/containerd/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)
See also Configure direct-lvm mode for production for additional information about production devmapper setups.
For more information on Device-mapper, thin provisioning, etc., you can refer to the following resources: