Use the BTRFS storage driver

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Btrfs is a next generation copy-on-write filesystem that supports many advanced storage technologies that make it a good fit for Docker. Btrfs is included in the mainline Linux kernel.

Docker’s btrfs storage driver leverages many Btrfs features for image and container management. Among these features are block-level operations, thin provisioning, copy-on-write snapshots, and ease of administration. You can easily combine multiple physical block devices into a single Btrfs filesystem.

This article refers to Docker’s Btrfs storage driver as btrfs and the overall Btrfs Filesystem as Btrfs.

Note: The btrfs storage driver is only supported on Docker CE on Ubuntu or Debian, and Docker EE / CS Engine on SLES.

Prerequisites

btrfs is supported if you meet the following prerequisites:

  • Docker CE: For Docker CE, btrfs is only recommended on Ubuntu or Debian.

  • Docker EE: For Docker EE and CS-Engine, btrfs is only supported on SLES. See the Product compatibility matrix for all supported configurations for commercially-supported Docker.

  • Changing the storage driver makes any containers you have already created inaccessible on the local system. Use docker save to save containers, and push existing images to Docker Hub or a private repository, so that you not need to re-create them later.

  • btrfs requires a dedicated block storage device such as a physical disk. This block device must be formatted for Btrfs and mounted into /var/lib/docker/. The configuration instructions below walk you through this procedure. By default, the SLES / filesystem is formatted with BTRFS, so for SLES, you do not need to use a separate block device, but you can choose to do so for performance reasons.

  • btrfs support must exist in your kernel. To check this, run the following command:

    $ sudo cat /proc/filesystems | grep btrfs

btrfs

  • To manage BTRFS filesystems at the level of the operating system, you need the btrfs command. If you do not have this command, install the btrfsprogs package (SLES) or btrfs-tools package (Ubuntu).

Configure Docker to use the btrfs storage driver

This procedure is essentially identical on SLES and Ubuntu.

  1. Stop Docker.

  2. Copy the contents of /var/lib/docker/ to a backup location, then empty the contents of /var/lib/docker/:

    $ sudo cp -au /var/lib/docker /var/lib/docker.bk
$ sudo rm -rf /var/lib/docker/*

  3. Format your dedicated block device or devices as a Btrfs filesystem. This example assumes that you are using two block devices called /dev/xvdf and /dev/xvdg. Double-check the block device names because this is a destructive operation.

    $ sudo mkfs.btrfs -f /dev/xvdf /dev/xvdg

    There are many more options for Btrfs, including striping and RAID. See the Btrfs documentation.

  4. Mount the new Btrfs filesystem on the /var/lib/docker/ mount point. You can specify any of the block devices used to create the Btrfs filesystem.

    $ sudo mount -t btrfs /dev/xvdf /var/lib/docker

    Don’t forget to make the change permanent across reboots by adding an entry to /etc/fstab.

  5. Copy the contents of /var/lib/docker.bk to /var/lib/docker/.

    $ sudo cp -au /var/lib/docker.bk/* /var/lib/docker/

  6. Configure Docker to use the btrfs storage driver. This is required even though /var/lib/docker/ is now using a Btrfs filesystem. Edit or create the file /etc/docker/daemon.json. If it is a new file, add the following contents. If it is an existing file, add the key and value only, being careful to end the line with a comma if it is not the final line before an ending curly bracket (}).

    {
  "storage-driver": "btrfs"
}

    See all storage options for each storage driver:

  7. Start Docker. After it is running, verify that btrfs is being used as the storage driver.

    $ docker info

Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 0
Server Version: 17.03.1-ce
Storage Driver: btrfs
 Build Version: Btrfs v4.4
 Library Version: 101
<output truncated>

  8. When you are ready, remove the /var/lib/docker.bk directory.

Manage a Btrfs volume

One of the benefits of Btrfs is the ease of managing Btrfs filesystems without the need to unmount the filesystem or restart Docker.

When space gets low, Btrfs automatically expands the volume in chunks of roughly 1 GB.

To add a block device to a Btrfs volume, use the btrfs device add and btrfs filesystem balance commands.

$ sudo btrfs device add /dev/svdh /var/lib/docker

$ sudo btrfs filesystem balance /var/lib/docker

Note: While you can do these operations with Docker running, performance suffers. It might be best to plan an outage window to balance the Btrfs filesystem.

How the btrfs storage driver works

The btrfs storage driver works differently from devicemapper or other storage drivers in that your entire /var/lib/docker/ directory is stored on a Btrfs volume.

Image and container layers on-disk

Information about image layers and writable container layers is stored in /var/lib/docker/btrfs/subvolumes/. This subdirectory contains one directory per image or container layer, with the unified filesystem built from a layer plus all its parent layers. Subvolumes are natively copy-on-write and have space allocated to them on-demand from an underlying storage pool. They can also be nested and snapshotted. The diagram below shows 4 subvolumes. ‘Subvolume 2’ and ‘Subvolume 3’ are nested, whereas ‘Subvolume 4’ shows its own internal directory tree.

subvolume example

Only the base layer of an image is stored as a true subvolume. All the other layers are stored as snapshots, which only contain the differences introduced in that layer. You can create snapshots of snapshots as shown in the diagram below.

snapshots diagram

On disk, snapshots look and feel just like subvolumes, but in reality they are much smaller and more space-efficient. Copy-on-write is used to maximize storage efficiency and minimize layer size, and writes in the container’s writable layer are managed at the block level. The following image shows a subvolume and its snapshot sharing data.

snapshot and subvolume sharing data

For maximum efficiency, when a container needs more space, it is allocated in chunks of roughly 1 GB in size.

Docker’s btrfs storage driver stores every image layer and container in its own Btrfs subvolume or snapshot. The base layer of an image is stored as a subvolume whereas child image layers and containers are stored as snapshots. This is shown in the diagram below.

Btrfs container layers

The high level process for creating images and containers on Docker hosts running the btrfs driver is as follows:

  1. The image’s base layer is stored in a Btrfs subvolume under /var/lib/docker/btrfs/subvolumes.

  2. Subsequent image layers are stored as a Btrfs snapshot of the parent layer’s subvolume or snapshot, but with the changes introduced by this layer. These differences are stored at the block level.

  3. The container’s writable layer is a Btrfs snapshot of the final image layer, with the differences introduced by the running container. These differences are stored at the block level.

How container reads and writes work with btrfs

Reading files

A container is a space-efficient snapshot of an image. Metadata in the snapshot points to the actual data blocks in the storage pool. This is the same as with a subvolume. Therefore, reads performed against a snapshot are essentially the same as reads performed against a subvolume.

Writing files

  • Writing new files: Writing a new file to a container invokes an allocate-on-demand operation to allocate new data block to the container’s snapshot. The file is then written to this new space. The allocate-on-demand operation is native to all writes with Btrfs and is the same as writing new data to a subvolume. As a result, writing new files to a container’s snapshot operates at native Btrfs speeds.

  • Modifying existing files: Updating an existing file in a container is a copy-on-write operation (redirect-on-write is the Btrfs terminology). The original data is read from the layer where the file currently exists, and only the modified blocks are written into the container’s writable layer. Next, the Btrfs driver updates the filesystem metadata in the snapshot to point to this new data. This behavior incurs very little overhead.

  • Deleting files or directories: If a container deletes a file or directory that exists in a lower layer, Btrfs masks the existence of the file or directory in the lower layer. If a container creates a file and then deletes it, this operation is performed in the Btrfs filesystem itself and the space is reclaimed.

With Btrfs, writing and updating lots of small files can result in slow performance.

Btrfs and Docker performance

There are several factors that influence Docker’s performance under the btrfs storage driver.

Note: Many of these factors are mitigated by using Docker volumes for write-heavy workloads, rather than relying on storing data in the container’s writable layer. However, in the case of Btrfs, Docker volumes still suffer from these draw-backs unless /var/lib/docker/volumes/ is not backed by Btrfs.

  • Page caching. Btrfs does not support page cache sharing. This means that each process accessing the same file copies the file into the Docker hosts’s memory. As a result, the btrfs driver may not be the best choice high-density use cases such as PaaS.

  • Small writes. Containers performing lots of small writes (this usage pattern matches what happens when you start and stop many containers in a short period of time, as well) can lead to poor use of Btrfs chunks. This can prematurely fill the Btrfs filesystem and lead to out-of-space conditions on your Docker host. Use btrfs filesys show to closely monitor the amount of free space on your Btrfs device.

  • Sequential writes. Btrfs uses a journaling technique when writing to disk. This can impact the performance of sequential writes, reducing performance by up to 50%.

  • Fragmentation. Fragmentation is a natural byproduct of copy-on-write filesystems like Btrfs. Many small random writes can compound this issue. Fragmentation can manifest as CPU spikes when using SSDs or head thrashing when using spinning disks. Either of these issues can harm performance.

    If your Linux kernel version is 3.9 or higher, you can enable the autodefrag feature when mounting a Btrfs volume. Test this feature on your own workloads before deploying it into production, as some tests have shown a negative impact on performance.

  • SSD performance: Btrfs includes native optimizations for SSD media. To enable these features, mount the Btrfs filesystem with the -o ssd mount option. These optimizations include enhanced SSD write performance by avoiding optimization such as seek optimizations which do not apply to solid-state media.

  • Balance Btrfs filesystems often: Use operating system utilities such as a cron job to balance the Btrfs filesystem regularly, during non-peak hours. This reclaims unallocated blocks and helps to prevent the filesystem from filling up unnecessarily. You cannot rebalance a totally full Btrfs filesystem unless you add additional physical block devices to the filesystem. See the BTRFS Wiki.

  • Use fast storage: Solid-state drives (SSDs) provide faster reads and writes than spinning disks.

  • Use volumes for write-heavy workloads: Volumes provide the best and most predictable performance for write-heavy workloads. This is because they bypass the storage driver and do not incur any of the potential overheads introduced by thin provisioning and copy-on-write. Volumes have other benefits, such as allowing you to share data among containers and persisting even when no running container is using them.

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