Docker stores images and containers in a graph driver (a pluggable storage
backend), such as Device Mapper, Overlay, and Btrfs. Each have advantages and
disadvantages. For example, Overlay is faster than Device Mapper at starting and
stopping containers, but is not POSIX compliant because of the architectural
limitations of a union file system, and does not yet support SELinux.
In production environments, using a LVM thin pool on top of regular block
devices (not loop devices) for container images and container root file systems
storage is recommended.
Using a Loop device back-end can affect performance issues. While you can still
continue to use it, Docker logs a warning message. For example:
devmapper: Usage of loopback devices is strongly discouraged for production use.
Please use `--storage-opt dm.thinpooldev` or use `man docker` to refer to
To ease Docker backend storage configuration, use the
docker-storage-setup utility, which automates much of the configuration
If you had a separate disk drive dedicated to Docker storage (for example,
/dev/xvdb), add the following to the /etc/sysconfig/docker-storage-setup
# systemctl restart docker-storage-setup
After the restart,
docker-storage-setup sets up a volume group named
docker_vg and creates a thin pool logical volume. Documentation for thin
provisioning on RHEL is available in the
Administrator Guide. View the newly created volumes with the
# lsblk /dev/xvdb
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvdb 202:16 0 20G 0 disk
└─xvdb1 202:17 0 10G 0 part
├─docker_vg-docker--pool_tmeta 253:0 0 12M 0 lvm
│ └─docker_vg-docker--pool 253:2 0 6.9G 0 lvm
└─docker_vg-docker--pool_tdata 253:1 0 6.9G 0 lvm
└─docker_vg-docker--pool 253:2 0 6.9G 0 lvm
Thin-provisioned volumes are not mounted and have no file system (individual
containers do have an XFS file system), thus they will not show up in “df”
To verify that Docker is using a LVM thin pool, and to monitor disk space
utilization, use the
docker info command. The
Pool Name will correspond with
VG you specified in /etc/sysconfig/docker-storage-setup:
# docker info | egrep -i 'storage|pool|space|filesystem'
Storage Driver: devicemapper
Pool Name: docker_vg-docker--pool
Pool Blocksize: 524.3 kB
Backing Filesystem: xfs
Data Space Used: 62.39 MB
Data Space Total: 6.434 GB
Data Space Available: 6.372 GB
Metadata Space Used: 40.96 kB
Metadata Space Total: 16.78 MB
Metadata Space Available: 16.74 MB
By default, a thin pool is configured to use 40% of the underlying block device.
As you use the storage, LVM automatically extends the thin pool up to 100%. This
is why the
Data Space Total value does not match the full size of the
underlying LVM device. This auto-extend technique was used to unify the storage
approach taken in both Red Hat Enterprise Linux and Red Hat Atomic Host, which
only uses a single partition.
In development, Docker in Red Hat distributions defaults to a
loopback mounted sparse file. To see if your system is using the loopback mode:
# docker info|grep loop0
Data file: /dev/loop0
Red Hat strongly recommends using the Device Mapper storage driver in thin pool
mode for production workloads.
Overlay is also supported for Docker use cases as of Red Hat Enterprise Linux
7.2, and provides faster start up time and page cache sharing, which can
potentially improve density by reducing overall memory utilization.