FUSE-based POSIX API

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The Alluxio POSIX API is a feature that allows mounting an Alluxio File System as a standard file system on most flavors of Unix. By using this feature, standard tools (for example, ls, cat or mkdir) will have basic access to the Alluxio namespace. More importantly, with the POSIX API integration applications can interact with the Alluxio no matter what language (C, C++, Python, Ruby, Perl, or Java) they are written in without any Alluxio library integrations.

Note that Alluxio-FUSE is different from projects like s3fs, mountableHdfs which mount specific storage services like S3 or HDFS to the local filesystem. The Alluxio POSIX API is a generic solution for the many storage systems supported by Alluxio. Data orchestration and caching features from Alluxio speed up I/O access to frequently used data.

Alluxio stack with its POSIX API

The Alluxio POSIX API is based on the Filesystem in Userspace (FUSE) project. 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 does not have full POSIX semantics and contains some 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 properly configuring and starting an Alluxio cluster; Run the following command on the node where you want to create the mount point:

$ ${ALLUXIO_HOME}/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, running the following commands from the ${ALLUXIO_HOME} directory will mount the Alluxio path /people to the folder /mnt/people on 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, the value defaults to the 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. Multiple Alluxio FUSE mount points can be created in the same node. All the 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 filesystem.

Unmount Alluxio-FUSE

To unmount a previously mounted Alluxio-FUSE file system, on the node where the file system is mounted run:

$ ${ALLUXIO_HOME}/integration/fuse/bin/alluxio-fuse unmount mount_point

This unmounts the file system at the mount point and stops the corresponding Alluxio-FUSE process. For example,

$ ${ALLUXIO_HOME}/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 mount points; on the node where the file system is mounted run:

$ ${ALLUXIO_HOME}/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 could be:

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 depending on the platform.

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

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

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, the 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-links or soft-links, so commands like ln are not supported. The hardlinks number is not 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. 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 for read or write operations. 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 the 3.15 Linux Kernel (supported by libfuse 3.x but not yet supported in jnr-fuse).

Configuration Parameters For Alluxio POSIX API

These are the configuration parameters for Alluxio POSIX API.

ParameterDefault ValueDescription
alluxio.fuse.cachedpaths.max 500
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.