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Optimizing Generic File System Sharing from Bind Mounts

Optimizing Generic File System Sharing from Bind Mounts


This document proposes an extension to bind mount semantics that unlocks significant performance improvements for osxfs. In preliminary measurements, go list running time drops from 25.7 seconds to 7.6 seconds.


With Docker distributions 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 and macOS 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 workload the overhead of maintaining perfect consistency can result in performance that is significantly worse than a native (non-Docker) environment. For example,

  • running go list ./... in a bind-mounted golang 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 over the last few months, and a few further optimization opportunities remain. However, even when latency is minimized, the constraints of maintaining consistency mean that these workloads are likely to remain unacceptably slow for some users.

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 artefacts within the container to be immediately reflected on the host file system. At present these two cases are treated identically, but distinguishing between them will allow us to significantly improve performance.

Several users are already using third-party solutions (e.g. rsync or asynchronous NFS) that offer improved performance for particular use cases at the cost of both consistency and other benefits such as proper handling of permissions and file system events. Allowing these users to select between perfect consistency and improved performance on a per-mount basis will greatly improve their Docker experience. We also anticipate being able to offer performance that surpasses NFS through aggressive caching.

There are three broad scenarios to consider. 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)

These three options are described in more detail below ("Semantics").

Preliminary performance improvements

Preliminary measurements reveal that that new configurations can offer immediate performance improvements.

In this section, 'preliminary' indicates that the implementation under test is only using aggressive Linux VFS caching features rather than any additional and significant structural caching opportunities enabled by the below semantics. All measurements were performed on a pinata branch from Beta 40. Parenthesized ratios represent the improvement in total running time over the consistent configuration. "With[out] disk image synchronization" refers to whether F_FULLFSYNC is used on the pinata qcow block device.

  • running go list ./... in a golang source tree (read-only workload):

    • without shared directory: 2.0s
    • with a consistent shared directory: 25.7s
    • with preliminary cached semantics cold: 14.4s (1.81×)
    • with preliminary cached semantics hot: 7.6s (3.38×)
  • performing many small writes (dd 100,000 1k blocks, write-only):

    • without disk image synchronization: 1.0s
    • with disk image synchronization: 1.6s
    • with a consistent shared directory: 22.7s
    • with preliminary delegated semantics: 1.9s (11.95×)
  • running ember build on a freshly created (i.e. empty) application

    • without disk image synchronization (unsafe): 10.1s
    • with disk image synchronization (default): 10.5s
    • with a consistent shared directory: 27.2s
    • with preliminary cached semantics cold: 22.4s (1.21×)
    • with preliminary cached semantics hot: 17.9s (1.52×)
    • with preliminary delegated semantics cold: 21.6s (1.26×)
    • with preliminary delegated semantics hot: 17.7s (1.54×)


The bind options are the natural place to support the selection of semantics. PR28527 lists the existing options (type, readonly, etc.).

We propose adding a single new bind option, state, with values consistent (the default), cached, and delegated. No additional interface changes are needed for this proposal, although broadening the interface (e.g. to specify per-mount policies around permissions, uids, etc.) might be useful in future. However, a number of additional improvements to the interface are discussed below ("Design and Development Considerations").

As the detailed semantics below show, ignoring the new option altogether is an acceptable implementation of the various semantics on Linux. This property enables cross-platform compatibility without any low-level changes in the Docker engine.


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 Semantics

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 artefacts) 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 run-time 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 run-time 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 time frame.

(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 Semantics

The cached configuration provides all the guarantees of the delegated configuration and some additional guarantees around the visibility of writes performed by containers. 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.

(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 Semantics

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 behaviour 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 Semantics

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.

Design and Development Considerations

Besides the additional bind option described above, users could ideally use a configuration file, extended attribute, application database, or Dockerfile to persistently specify mount sources that should always be associated with certain consistency properties. Without this capability, users must either each learn of these flags or rely solely on scripts that wrap the Docker command line and provide these flags. Additionally, many common developer use cases such as mounting a source code directory would benefit from mounts that use different consistency properties on different subdirectories. For example, project build directories could likely be delegated when running a build monitoring container whereas source, storage, or control directories should not be delegated.

Finally, the delegated mode requires the ability to interpose on a container shutdown sequence and potentially alter its success status. The capability may be present in runc or containerd but it is not clear how or if it is exposed to the Docker run-time.


With minimal client change, the cached and delegated mount flags offer the potential for large speedups of shared file system performance.


Please contact and with questions, concerns, suggestions, or additional resources.

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