FUSE-based POSIX API

Alluxio-FUSE 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 FUSE, an application written in any language like C, C++, Python, Ruby, Perl, or Java, can interact with Alluxio by using standard POSIX APIs like open, write, read, without any Alluxio client integration or set up.

Alluxio-FUSE 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.

integration/fuse/bin/alluxio-fuse mount /mnt/people /people
Starting alluxio-fuse on local host. Alluxio-fuse mounted at /mnt/people. See /lib/alluxio/logs/fuse.log for logs

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 alluxio-fuse.sh 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 alluxio-fuse 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 umount a previously mounted Alluxio-FUSE file sytem, on the node where the file system is mounted, point a shell to your $ALLUXIO_HOME and run:

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

Check the Alluxio-FUSE 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-FUSE 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-FUSE 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 before Alluxio-FUSE starts.

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 commonly used mount options are listed here.

$ 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. If you want to allow other users or allow root to access the mounted folder, you can add the following line to the file /etc/fuse.conf:

user_allow_other

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

After that, you can 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.
• 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 Fuse 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-FUSE

These are the configuration parameters for Alluxio-FUSE.

Acknowledgements

This project uses jnr-fuse for FUSE on Java.