Performance tuning for volume mounts (shared filesystems)

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Docker 17.04 CE Edge adds support for two new flags to the docker run -v, --volume option, cached and delegated, that can significantly improve the performance of mounted volume access on Docker for Mac. These options begin to solve some of the challenges discussed in Performance issues, solutions, and roadmap.

Tip: Release notes for Docker CE Edge 17.04 are here, and the associated pull request for the additional docker run -v flags is here.

Performance implications of host-container file system consistency

With Docker distributions now available for an increasing number of platforms, including macOS and Windows, generalizing mount semantics during container run is a necessity to enable workload optimizations.

The current implementations of mounts on Linux provide a consistent view of a host directory tree inside a container: reads and writes performed either on the host or in the container are immediately reflected in the other environment, and file system events (inotify, FSEvents) are consistently propagated in both directions.

On Linux, these guarantees carry no overhead, since the underlying VFS is shared directly between host and container. However, on macOS (and other non-Linux platforms) there are significant overheads to guaranteeing perfect consistency, since messages describing file system actions must be passed synchronously between container and host. The current implementation is sufficiently efficient for most tasks, but with certain types of workloads the overhead of maintaining perfect consistency can result in significantly worse performance than a native (non-Docker) environment. For example,

  • running go list ./... in the bind-mounted docker/docker source tree takes around 26 seconds

  • writing 100MB in 1k blocks into a bind-mounted directory takes around 23 seconds

  • running ember build on a freshly created (i.e. empty) application involves around 70000 sequential syscalls, each of which translates into a request and response passed between container and host.

Optimizations to reduce latency throughout the stack have brought significant improvements to these workloads, and a few further optimization opportunities remain. However, even when latency is minimized, the constraints of maintaining consistency mean that these workloads remain unacceptably slow for some use cases.

Tuning with consistent, cached, and delegated configurations

Fortunately, in many cases where the performance degradation is most severe, perfect consistency between container and host is unnecessary. In particular, in many cases there is no need for writes performed in a container to be immediately reflected on the host. For example, while interactive development requires that writes to a bind-mounted directory on the host immediately generate file system events within a container, there is no need for writes to build artifacts within the container to be immediately reflected on the host file system. Distinguishing between these two cases makes it possible to significantly improve performance.

There are three broad scenarios to consider, based on which you can dial in the level of consistency you need. In each case, the container has an internally-consistent view of bind-mounted directories, but in two cases temporary discrepancies are allowed between container and host.

  • consistent: perfect consistency
    (host and container have an identical view of the mount at all times)

  • cached: the host’s view is authoritative
    (permit delays before updates on the host appear in the container)

  • delegated: the container’s view is authoritative
    (permit delays before updates on the container appear in the host)

Examples

Each of these configurations (consistent, cached, delegated) can be specified as a suffix to the -v option of docker run. For example, to bind-mount /Users/yallop/project in a container under the path /project, you might run the following command:

docker run -v /Users/yallop/project:/project:cached alpine command

The caching configuration can be varied independently for each bind mount, so you can mount each directory in a different mode:

docker run -v /Users/yallop/project:/project:cached \
 -v /host/another-path:/mount/another-point:consistent
 alpine command

Semantics

The semantics of each configuration is described as a set of guarantees relating to the observable effects of file system operations. In this specification, “host” refers to the file system of the user’s Docker client.

delegated

The delegated configuration provides the weakest set of guarantees. For directories mounted with delegated the container’s view of the file system is authoritative, and writes performed by containers may not be immediately reflected on the host file system. As with (e.g.) NFS asynchronous mode, if a running container with a delegated bind mount crashes, then writes may be lost.

However, by relinquishing consistency, delegated mounts can offer significantly better performance than the other configurations. Where the data written is ephemeral or readily reproducible (e.g. scratch space or build artifacts) delegated may be optimal for a user’s workload.

A delegated mount offers the following guarantees, which are presented as constraints on the container run-time:

  1. If the implementation offers file system events, the container state as it relates to a specific event must reflect the host file system state at the time the event was generated if no container modifications pertain to related file system state.

  2. If flush or sync operations are performed, relevant data must be written back to the host file system. Between flush or sync operations containers may cache data written, metadata modifications, and directory structure changes.

  3. All containers hosted by the same runtime must share a consistent cache of the mount.

  4. When any container sharing a delegated mount terminates, changes to the mount must be written back to the host file system. If this writeback fails, the container’s execution must fail via exit code and/or Docker event channels.

  5. If a delegated mount is shared with a cached or a consistent mount, those portions that overlap must obey cached or consistent mount semantics, respectively.

    Besides these constraints, the delegated configuration offers the container runtime a degree of flexibility:

  6. Containers may retain file data and metadata (including directory structure, existence of nodes, etc) indefinitely and this cache may desynchronize from the file system state of the host. Implementors are encouraged to expire caches when host file system changes occur but, due to platform limitations, may be unable to do this in any specific timeframe.

  7. If changes to the mount source directory are present on the host file system, those changes may be lost when the delegated mount synchronizes with the host source directory.

  8. Behaviors 6-7 do not apply to the file types of socket, pipe, or device.

cached

The cached configuration provides all the guarantees of the delegated configuration, and some additional guarantees around the visibility of writes performed by containers. As such, cached typically improves the performance of read-heavy workloads, at the cost of some temporary inconsistency between the host and the container.

For directories mounted with cached, the host’s view of the file system is authoritative; writes performed by containers are immediately visible to the host, but there may be a delay before writes performed on the host are visible within containers.

Tip: To learn more about cached, see the article on User-guided caching in Docker for Mac.

  1. Implementations must obey delegated Semantics 1-5.

  2. If the implementation offers file system events, the container state as it relates to a specific event must reflect the host file system state at the time the event was generated.

  3. Container mounts must perform metadata modifications, directory structure changes, and data writes consistently with the host file system, and must not cache data written, metadata modifications, or directory structure changes.

  4. If a cached mount is shared with a consistent mount, those portions that overlap must obey consistent mount semantics.

    Some of the flexibility of the delegated configuration is retained, namely:

  5. Implementations may permit delegated Semantics 6.

consistent

The consistent configuration places the most severe restrictions on the container run-time. For directories mounted with consistent the container and host views are always synchronized: writes performed within the container are immediately visible on the host, and writes performed on the host are immediately visible within the container.

The consistent configuration most closely reflects the behavior of bind mounts on Linux. However, the overheads of providing strong consistency guarantees make it unsuitable for a few use cases, where performance is a priority and maintaining perfect consistency has low priority.

  1. Implementations must obey cached Semantics 1-4.

  2. Container mounts must reflect metadata modifications, directory structure changes, and data writes on the host file system immediately.

default

The default configuration is identical to the consistent configuration except for its name. Crucially, this means that cached Semantics 4 and delegated Semantics 5 that require strengthening overlapping directories do not apply to default mounts. This is the default configuration if no state flags are supplied.

mac, osxfs, volumes