FUSE-based POSIX API

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Alluxio POSIX API is a feature that allows mounting the distributed Alluxio File System as a standard file system on most flavors of Unix. By using this feature, standard bash tools (for example, ls, cat or mkdir) will have basic access to the distributed Alluxio data store. More importantly, with this POSIX API, applications which can interact with the local filesystem, no matter what languages (C, C++, Python, Ruby, Perl, or Java) they are written in, can interact with Alluxio and its under storages without any Alluxio client integration or set up.

Note that, different from projects like s3fs, mountableHdfs which can mount specific storage service like S3 or HDFS as local filesystem, the Alluxio POSIX API is a generic solution for all storage systems supported by Alluxio. The rich data orchestration and caching service inherited from Alluxio speeds up the I/O access to frequently used data in Alluxio worker memory space.

Alluxio stack with its POSIX API

Alluxio POSIX API is based on the project Filesystem in Userspace (FUSE), and most basic file system operations are supported. However, given the intrinsic characteristics of Alluxio, like its write-once/read-many-times file data model, the mounted file system will not have full POSIX semantics and will have specific limitations. Please read the section of limitations for details.

Requirements

  • JDK 1.8 or newer
  • libfuse 2.9.3 or newer (2.8.3 has been reported to also work - with some warnings) for Linux
  • osxfuse 3.7.1 or newer for MacOS

Usage

Mount Alluxio-FUSE

After having properly configured and started the Alluxio cluster, and from the node where you wish to mount Alluxio, point a shell to your $ALLUXIO_HOME and run:

$ integration/fuse/bin/alluxio-fuse mount mount_point [alluxio_path]

This will spawn a background user-space java process (alluxio-fuse) that will mount the Alluxio path specified at alluxio_path to the local file system on the specified mount_point.

For example, the following command will mount the Alluxio path /people to the folder /mnt/people in the local file system.

$ ./bin/alluxio fs mkdir /people
$ sudo mkdir -p /mnt/people
$ sudo chown $(whoami) /mnt/people
$ chmod 755 /mnt/people
$ integration/fuse/bin/alluxio-fuse mount /mnt/people /people

When alluxio_path is not given, Alluxio-FUSE defaults it to root (/). Note that the mount_point must be an existing and empty path in your local file system hierarchy and that the user that runs the integration/fuse/bin/alluxio-fuse script must own the mount point and have read and write permissions on it. You can mount Alluxio to multiple mount points. All of these AlluxioFuse processes share the same log output at $ALLUXIO_HOME\logs\fuse.log, which is useful for troubleshooting when errors happen on operations under the mounting point.

Unmount Alluxio-FUSE

To unmount a previously mounted Alluxio-FUSE file system, on the node where the file system is mounted, point a shell to your $ALLUXIO_HOME and run:

$ integration/fuse/bin/alluxio-fuse unmount mount_point

This unmounts the file system at the mounting point and stops the corresponding alluxio-fuse process. For example,

$ integration/fuse/bin/alluxio-fuse unmount /mnt/people
Unmount fuse at /mnt/people (PID:97626).

Check the Alluxio POSIX API mounting status

To list the mounting points, on the node where the file system is mounted, point a shell to your $ALLUXIO_HOME and run:

$ integration/fuse/bin/alluxio-fuse stat

This outputs the pid, mount_point, alluxio_path of all the running Alluxio-FUSE processes.

For example, the output will be like:

pid	mount_point	alluxio_path
80846	/mnt/people	/people
80847	/mnt/sales	/sales

Advanced configuration

Configure Alluxio client options

Alluxio POSIX API is based on the standard Java client API alluxio-core-client-fs to perform its operations. You might want to customize the behaviour of the Alluxio client used by Alluxio POSIX API the same way you would for any other client application.

One possibility, for example, is to edit $ALLUXIO_HOME/conf/alluxio-site.properties and set your specific Alluxio client options. Note that these changes should be done before the mounting steps.

Configure mount point options

You can use -o [mount options] to set mount options. If you want to set multiple mount options, you can pass in comma separated mount options as the value of -o. The -o [mount options] must follow the mount command.

The available Linux mount options are listed here. The mount options of MacOS with osxfuse are listed here . Some mount options (e.g. allow_other and allow_root) need additional set-up and the set up process may be different according to platforms.

$ integration/fuse/bin/alluxio-fuse mount \
  -o [comma separated mount options] mount_point [alluxio_path]

Note that direct_io mount option is set by default so that writes and reads bypass the kernel page cache and go directly to Alluxio.

Note that different versions of libfuse and osxfuse support different mount options.

Example: allow_other or allow_root

By default, Alluxio Fuse mount point can only be accessed by the user mounting the Alluxio namespace to the local filesystem.

For Linux, add the following line to file /etc/fuse.conf to allow other users or allow root to access the mounted folder:

user_allow_other

This option allow non-root users to specify the allow_other or allow_root mount options.

For MacOS, follow the osxfuse allow_other instructions to allow other users to use the allow_other and allow_root mount options.

After setting up, pass the allow_other or allow_root mount options when mounting Alluxio-Fuse:

# All users (including root) can access the files.
$ integration/fuse/bin/alluxio-fuse mount -o allow_other mount_point [alluxio_path]
# The user mounting the filesystem and root can access the files.
$ integration/fuse/bin/alluxio-fuse mount -o allow_root mount_point [alluxio_path]

Note that only one of the allow_other or allow_root could be set.

Assumptions and limitations

Currently, most basic file system operations are supported. However, due to Alluxio implicit characteristics, please be aware that:

  • Files can be written only once, only sequentially, and never be modified. That means overriding a file is not allowed, and an explicit combination of delete and then create is needed. For example, cp command will fail when the destination file exists. vi and vim commands will succeed because the underlying system do create, delete, and rename operation combinations.
  • Alluxio does not have hard-link and soft-link concepts, so the commands like ln are not supported, neither the hardlinks number is displayed in ll output.
  • The user and group are mapped to the Unix user and group only when Alluxio POSIX API is configured to use shell user group translation service, by setting alluxio.fuse.user.group.translation.enabled to true in conf/alluxio-site.properties. Otherwise chown and chgrp are no-ops, and ll will return the user and group of the user who started the Alluxio-FUSE process. The translation service does not change the actual file permission when running ll.

Performance considerations

Due to the conjunct use of FUSE and JNR, the performance of the mounted file system is expected to be worse than what you would see by using the Alluxio Java client directly.

Most of the overheads come from the fact that there are several memory copies going on for each call on read or write operations, and that FUSE caps the maximum granularity of writes to 128KB. This could be probably improved by a large extent by leveraging the FUSE cache write-backs feature introduced in kernel 3.15 (supported by libfuse 3.x userspace libs but not supported in jnr-fuse yet).

Configuration Parameters For Alluxio POSIX API

These are the configuration parameters for Alluxio POSIX API.

ParameterDefault ValueDescription
alluxio.fuse.cachedpaths.max 500 Defines the size of the internal Alluxio-FUSE cache that maintains the most frequently used translations between local file system paths and Alluxio file URIs.
alluxio.fuse.debug.enabled false Enable FUSE debug output. This output will be redirected in a `fuse.out` log file inside `alluxio.logs.dir`.
alluxio.fuse.fs.name alluxio-fuse Descriptive name used by FUSE to mount the file system.
alluxio.fuse.maxwrite.bytes 131072 The desired granularity of FUSE write upcalls in bytes. Note that 128K is currently an upper bound imposed by the linux kernel.
alluxio.fuse.user.group.translation.enabled false Whether to translate Alluxio users and groups into Unix users and groups when exposing Alluxio files through the FUSE API. When this property is set to false, the user and group for all FUSE files will match the user who started the alluxio-fuse process

Acknowledgements

This project uses jnr-fuse for FUSE on Java.