File system sharing (osxfs)

osxfs is a new shared file system solution, exclusive to Docker Desktop for Mac. osxfs provides a close-to-native user experience for bind mounting macOS file system trees into Docker containers. To this end, osxfs features a number of unique capabilities as well as differences from a classical Linux file system.

Case sensitivity

With Docker Desktop for Mac, file systems operate in containers in the same way as they operate in macOS. If a file system on macOS is case-insensitive, that behavior is shared by any bind mount from macOS into a container.

On macOS Sierra and lower, the default file system is HFS+. On macOS High Sierra, the default file system is APFS. Both are case-insensitive by default but available in case-sensitive and case-insensitive variants.

To get case-sensitive behavior, format the volume used in your bind mount as HFS+ or APFS with case-sensitivity. See the APFS FAQ.

Reformatting your root partition is not recommended as some Mac software relies on case-insensitivity to function.

Access control

osxfs, and therefore Docker, can access only those file system resources that the Docker Desktop for Mac user has access to. osxfs does not run as root. If the macOS user is an administrator, osxfs inherits those administrator privileges. We are still evaluating which privileges to drop in the file system process to balance security and ease-of-use. osxfs performs no additional permissions checks and enforces no extra access control on accesses made through it. All processes in containers can access the same objects in the same way as the Docker user who started the containers.

Namespaces

Much of the macOS file system that is accessible to the user is also available to containers using the -v bind mount syntax. The following command runs a container from an image called r-base and shares the macOS user’s ~/Desktop/ directory as /Desktop in the container.

$ docker run -it -v ~/Desktop:/Desktop r-base bash

The user’s ~/Desktop/ directory is now visible in the container as a directory under /.

root@2h30fa0c600e:/# ls
Desktop	boot	etc	lib	lib64	media	opt	root	sbin	sys	usr
bin	dev	home	lib32	libx32	mnt	proc	run	srv	tmp	var

By default, you can share files in /Users/, /Volumes/, /private/, and /tmp directly. To add or remove directory trees that are exported to Docker, use the File sharing tab in Docker preferences -> Preferences -> File sharing. (See Preferences.)

All other paths used in -v bind mounts are sourced from the Moby Linux VM running the Docker containers, so arguments such as -v /var/run/docker.sock:/var/run/docker.sock should work as expected. If a macOS path is not shared and does not exist in the VM, an attempt to bind mount it fails rather than create it in the VM. Paths that already exist in the VM and contain files are reserved by Docker and cannot be exported from macOS.

See Performance tuning for volume mounts (shared filesystems) to learn about new configuration options available with the Docker 17.04 CE Edge release.

Ownership

Initially, any containerized process that requests ownership metadata of an object is told that its uid and gid own the object. When any containerized process changes the ownership of a shared file system object, such as by using the chown command, the new ownership information is persisted in the com.docker.owner extended attribute of the object. Subsequent requests for ownership metadata return the previously set values. Ownership-based permissions are only enforced at the macOS file system level with all accessing processes behaving as the user running Docker. If the user does not have permission to read extended attributes on an object (such as when that object’s permissions are 0000), osxfs attempts to add an access control list (ACL) entry that allows the user to read and write extended attributes. If this attempt fails, the object appears to be owned by the process accessing it until the extended attribute is readable again.

File system events

Most inotify events are supported in bind mounts, and likely dnotify and fanotify (though they have not been tested) are also supported. This means that file system events from macOS are sent into containers and trigger any listening processes there.

The following are supported file system events:

• Creation
• Modification
• Attribute changes
• Deletion
• Directory changes

The following are partially supported file system events:

• Move events trigger IN_DELETE on the source of the rename and IN_MODIFY on the destination of the rename

The following are unsupported file system events:

• Open
• Access
• Close events
• Unmount events (see Mounts)

Some events may be delivered multiple times. These limitations do not apply to events between containers, only to those events originating in macOS.

Mounts

The macOS mount structure is not visible in the shared volume, but volume contents are visible. Volume contents appear in the same file system as the rest of the shared file system. Mounting/unmounting macOS volumes that are also bind mounted into containers may result in unexpected behavior in those containers. Unmount events are not supported. Mount export support is planned but is still under development.

Symlinks

Symlinks are shared unmodified. This may cause issues when symlinks contain paths that rely on the default case-insensitivity of the default macOS file system.

File types

Symlinks, hardlinks, socket files, named pipes, regular files, and directories are supported. Socket files and named pipes only transmit between containers and between macOS processes -- no transmission across the hypervisor is supported, yet. Character and block device files are not supported.

Extended attributes

Extended attributes are not yet supported.

Technology

osxfs does not use OSXFUSE. osxfs does not run under, inside, or between macOS userspace processes and the macOS kernel.

Performance issues, solutions, and roadmap

See Performance tuning for volume mounts (shared filesystems) to learn about new configuration options available with the Docker 17.04 CE Edge release.

With regard to reported performance issues (GitHub issue 77: File access in mounted volumes extremely slow), and a similar thread on Docker Desktop for Mac forums on topic: File access in mounted volumes extremely slow, this topic provides an explanation of the issues, recent progress in addressing them, how the community can help us, and what you can expect in the future. This explanation derives from a post about understanding performance by David Sheets (@dsheets) on the Docker development team to the forum topic just mentioned. We want to surface it in the documentation for wider reach.

Understanding performance

Perhaps the most important thing to understand is that shared file system performance is multi-dimensional. This means that, depending on your workload, you may experience exceptional, adequate, or poor performance with osxfs, the file system server in Docker Desktop for Mac. File system APIs are very wide (20-40 message types) with many intricate semantics involving on-disk state, in-memory cache state, and concurrent access by multiple processes. Additionally, osxfs integrates a mapping between macOS’s FSEvents API and Linux’s inotify API which is implemented inside of the file system itself, complicating matters further (cache behavior in particular).

At the highest level, there are two dimensions to file system performance: throughput (read/write IO) and latency (roundtrip time). In a traditional file system on a modern SSD, applications can generally expect throughput of a few GB/s. With large sequential IO operations, osxfs can achieve throughput of around 250 MB/s which, while not native speed, is not likely to be the bottleneck for most applications which perform acceptably on HDDs.

Latency is the time it takes for a file system call to complete. For instance, the time between a thread issuing write in a container and resuming with the number of bytes written. With a classical block-based file system, this latency is typically under 10μs (microseconds). With osxfs, latency is presently around 130μs for most operations or 13× slower. For workloads which demand many sequential roundtrips, this results in significant observable slowdown. Reducing the latency requires shortening the data path from a Linux system call to macOS and back again. This requires tuning each component in the data path in turn -- some of which require significant engineering effort. Even if we achieve a huge latency reduction of 65μs/roundtrip, we still “only” see a doubling of performance. This is typical of performance engineering, which requires significant effort to analyze slowdowns and develop optimized components. We know a number of approaches that may reduce the roundtrip time but we haven’t implemented all those improvements yet (more on this below in What you can do).

A second approach to improving performance is to reduce the number of roundtrips by caching data. Recent versions of Docker Desktop for Mac (17.04 onwards) include caching support that brings significant (2-4×) improvements to many applications. Much of the overhead of osxfs arises from the requirement to keep the container’s and the host’s view of the file system consistent, but full consistency is not necessary for all applications and relaxing the constraint opens up a number of opportunities for improved performance.

At present there is support for read caching, with which the container’s view of the file system can temporarily drift apart from the authoritative view on the host. Further caching developments, including support for write caching, are planned. A detailed description of the behavior in various caching configurations is available.

What we are doing

We continue to actively work on increasing caching and on reducing the file system data path latency. This requires significant analysis of file system traces and speculative development of system improvements to try to address specific performance issues. Perhaps surprisingly, application workload can have a huge effect on performance. As an example, here are two different use cases contributed on the forum topic and how their performance differs and suffers due to latency, caching, and coherence:

1. A rake example (see below) appears to attempt to access 37000+ different files that don’t exist on the shared volume. Even with a 2× speedup via latency reduction this use case still seems “slow”. With caching enabled the performance increases around 3.5×, as described in the user-guided caching post. We expect to see further performance improvements for rake with a “negative dcache” that keeps track of, in the Linux kernel itself, the files that do not exist. However, even this is not sufficient for the first time rake is run on a shared directory. To handle that case, we actually need to develop a Linux kernel patch which negatively caches all directory entries not in a specified set -- and this cache must be kept up-to-date in real-time with the macOS file system state even in the presence of missing macOS FSEvents messages and so must be invalidated if macOS ever reports an event delivery failure.

2. Running ember build in a shared file system results in ember creating many different temporary directories and performing lots of intermediate activity within them. An empty ember project is over 300MB. This usage pattern does not require coherence between Linux and macOS, and is significantly improved by write caching.

These two examples come from performance use cases contributed by users and they are incredibly helpful in prioritizing aspects of file system performance to improve. We are developing statistical file system trace analysis tools to characterize slow-performing workloads more easily to decide what to work on next.

Under development, we have:

1. A growing performance test suite of real world use cases (more on this below in What you can do)

2. Further caching improvements, including negative, structural, and write-back caching, and lazy cache invalidation.

3. A Linux kernel patch to reduce data path latency by 2/7 copies and 2/5 context switches

4. Increased macOS integration to reduce the latency between the hypervisor and the file system server

What you can do

When you report shared file system performance issues, it is most helpful to include a minimal Real World reproduction test case that demonstrates poor performance.

Without a reproduction, it is very difficult for us to analyze your use case and determine what improvements would speed it up. When you don’t provide a reproduction, one of us needs to figure out the specific software you are using and guess and hope that we have configured it in a typical way or a way that has poor performance. That usually takes 1-4 hours depending on your use case and once it is done, we must then determine what regular performance is like and what kind of slow-down your use case is experiencing. In some cases, it is not obvious what operation is even slow in your specific development workflow. The additional set-up to reproduce the problem means we have less time to fix bugs, develop analysis tools, or improve performance. So, include simple, immediate performance issue reproduction test cases. The rake reproduction case by @hirowatari shown in the forums thread is a great example.

This example originally provided:

1. A version-controlled repository so any changes/improvements to the test case can be easily tracked.

2. A Dockerfile which constructs the exact image to run

3. A command-line invocation of how to start the container

4. A straight-forward way to measure the performance of the use case

5. A clear explanation (README) of how to run the test case

What you can expect

We continue to work toward an optimized shared file system implementation on the Edge channel of Docker Desktop for Mac.

You can expect some of the performance improvement work mentioned above to reach the Edge channel in the coming release cycles.

We plan to eventually open source all of our shared file system components. At that time, we would be very happy to collaborate with you on improving the implementation of osxfs and related software.

We also plan to write up and publish further details of shared file system performance analysis and improvement on the Docker blog. Look for or nudge @dsheets about those articles, which should serve as a jumping off point for understanding the system, measuring it, or contributing to it.

Wrapping Up

We hope this gives you a rough idea of where osxfs performance is and where it’s going. We are treating good performance as a top priority feature of the file system sharing component and we are actively working on improving it through a number of different avenues. The osxfs project started in December

1. Since the first integration into Docker Desktop for Mac in February 2016, we’ve improved performance by 50x or more for many workloads while achieving nearly complete POSIX compliance and without compromising coherence (it is shared and not simply synced). Of course, in the beginning there was lots of low-hanging fruit and now many of the remaining performance improvements require significant engineering work on custom low-level components.

We appreciate your understanding as we continue development of the product and work on all dimensions of performance. We want to continue to work with the community on this, so continue to report issues as you find them. We look forward to collaborating with you on ideas and on the source code itself.