Unified Namespace

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This page summarizes how to manage different under storage systems in Alluxio namespace.


Alluxio enables effective data management across different storage systems through its use of transparent naming and mounting API.

Unified namespace

One of the key benefits that Alluxio provides is a unified namespace to the applications. This is an abstraction that allows applications to access multiple independent storage systems through the same namespace and interface. Rather than communicating to each individual storage system, applications can delegate this responsibility by connecting through Alluxio, which will handle the different underlying storage systems.


The directory specified by the alluxio.master.mount.table.root.ufs is mounted to the root of the Alluxio namespace. This directory identifies the “primary storage” for Alluxio. In addition, users can use the mounting API to add and remove data sources:

void mount(AlluxioURI alluxioPath, AlluxioURI ufsPath);
void mount(AlluxioURI alluxioPath, AlluxioURI ufsPath, MountOptions options);
void unmount(AlluxioURI path);
void unmount(AlluxioURI path, UnmountOptions options);

For example, mount a S3 bucket to the Data directory through

mount(new AlluxioURI("alluxio://host:port/Data"), new AlluxioURI("s3://bucket/directory"));

UFS namespace

In addition to the unified namespace Alluxio provides, each underlying file system that is mounted in Alluxio namespace has its own namespace; this is referred to as the UFS namespace. If a file in the UFS namespace is changed without going through Alluxio, the UFS namespace and the Alluxio namespace can potentially get out of sync. When this happens, a UFS Metadata Sync operation is required to synchronize the two namespaces.

Transparent Naming

Transparent naming maintains an identity between the Alluxio namespace and the underlying storage system namespace.


When a user creates objects in the Alluxio namespace, they can choose whether these objects should be persisted in the underlying storage system. For objects that are persisted, Alluxio preserves the object paths, relative to the underlying storage system directory in which Alluxio objects are stored. For instance, if a user creates a top-level directory Users with subdirectories Alice and Bob, the directory structure and naming is preserved in the underlying storage system. Similarly, when a user renames or deletes a persisted object in the Alluxio namespace, it is renamed or deleted in the underlying storage system.

Alluxio transparently discovers content present in the underlying storage system which was not created through Alluxio. For instance, if the underlying storage system contains a directory Data with files Reports and Sales, all of which were not created through Alluxio, their metadata will be loaded into Alluxio the first time they are accessed, such as when a user requests to open a file. The data of the file is not loaded to Alluxio during this process. To load the data into Alluxio, one can read the data using FileInStream or use the load command of the Alluxio shell.

Mounting Under Storage Systems

Mounting in Alluxio works similarly to mounting a volume in a Linux file system. The mount command attaches a UFS to the file system tree in the Alluxio namespace.

Root Mount Point

The root mount point of the Alluxio namespace can be specified in conf/alluxio-site.properties. The following line is an example configuration where a HDFS path is at the root of the Alluxio namespace.


Mount options for the root mount point can be configured using the configuration prefix:

alluxio.master.mount.table.root.option.<some alluxio property>

The following configuration adds AWS credentials for the root mount point.


The following configuration shows how to set other parameters for the root mount point.


Nested Mount Points

In addition to the root mount point, other under file systems can be mounted into Alluxio namespace by using the mount command. The --option flag allows the user to pass additional parameters to the mount operation, such as credentials for S3 storage.

$ ./bin/alluxio fs mount /mnt/hdfs hdfs://host1:9000/data/
$ ./bin/alluxio fs mount \
  --option aws.accessKeyId=<accessKeyId> --option aws.secretKey=<secretKey> \
  /mnt/s3 s3://data-bucket/

Note that mount points can be nested as well. For example, if a UFS is mounted at alluxio:///path1, another UFS could be mounted at alluxio:///path1/path2.

Mount UFS with Specific Versions

Alluxio supports mounting HDFS with specified versions. As a result, users can mount HDFS with different versions into a single Alluxio namespace. Please refer to HDFS Under Store for more details.

Relationship Between Alluxio and UFS Namespace

Alluxio provides a unified namespace, acting as a cache for data in one or more under file storage systems. This section discusses how Alluxio interacts with under file systems to discover their files and make them available through Alluxio.

Accessing UFS files through Alluxio should behave the same as accessing them directly through the UFS. If the root UFS is s3://bucket/data, listing alluxio:/// should show the same result as listing s3://bucket/data. Running cat on alluxio:///file should show the same result as running cat on s3://bucket/data/file.

Alluxio loads metadata from the UFS as needed. In the above example, Alluxio does not have information about s3://bucket/data/file at startup. The file will be discovered when the user lists alluxio:/// or tries to cat alluxio:///file. This prevents unnecessary work when mounting a new UFS.

By default, Alluxio expects that all modifications to under file systems occur through Alluxio. This allows Alluxio to only scan each UFS directory once, significantly improving performance when UFS metadata operations are slow. In the scenario where changes are made to the UFS outside of Alluxio, the metadata sync feature is used to synchronize the two namespaces.

UFS Metadata Sync

The UFS metadata sync feature has been available since version 1.7.0.

When Alluxio scans a UFS directory and loads metadata for its sub-paths, it creates a copy of the metadata so that future operations do not need to load from the UFS. By default, the cached copy lives forever but its lifetime can be configured using the alluxio.user.file.metadata.sync.interval property. This property applies to client side operations. For example, if a client executes an operation with the interval set to one minute, all relevant metadata must be synced within the past minute. A value of 0 indicates that the metadata will always be synced for every operation, whereas the default value of -1 indicates the metadata is never synced again after the initial load.

Low interval values allow Alluxio clients to quickly discover external modifications to the UFS, at the cost of decreasing performance since the number of calls to the UFS increases.

Metadata sync keeps a fingerprint of each UFS file so that Alluxio can update the file if it changes. The fingerprint includes information such as file size and last modified time. If a file is modified in the UFS, Alluxio will detect this from the fingerprint, free the existing data for that file, and reload the metadata for the updated file. If a file is added or deleted in the UFS, Alluxio will update the metadata in its namespace as well.

Techniques for managing UFS sync

Periodic Metadata Sync

If the UFS updates at a scheduled interval, you can manually trigger the sync command after the update. Set the sync interval to 0 by running the command:

$ ./bin/alluxio fs ls -R -Dalluxio.user.file.metadata.sync.interval=0

Then reset the sync interval back to the default value of -1, since it is known that there will be no further external changes to the UFS until the next update.

Centralized Configuration

For clusters where jobs run with data from a frequently updated UFS, it is inconvenient for every client to specify a sync interval. Requests will be handled with a default sync interval if one is set in the master configuration.

In alluxio-site.properties on master nodes:


Note master nodes need to be restarted to pick up configuration changes.

Other Methods for Loading New UFS Files

The UFS sync discussed previously is the recommended method for loading files from the UFS. Here are some other methods for loading files:

  • alluxio.user.file.metadata.load.type: This property can be set to either ALWAYS, ONCE, or NEVER. It acts similar to alluxio.user.file.metadata.sync.interval, but with two caveats:
    1. It only discovers new files and does not reload modified or deleted files.
    2. It only applies to the exists, list, and getStatus RPCs.

    ALWAYS will always check the UFS for new files, ONCE will use the default behavior of only scanning each directory once ever, and NEVER will prevent Alluxio from scanning for new files at all.

  • alluxio fs ls -f /path: The -f option to ls acts the same as setting alluxio.user.file.metadata.load.type to ALWAYS. It discovers new files but does not detect modified or deleted UFS files. The only way to detect these types of changes is to pass the -Dalluxio.user.file.metadata.sync.interval=0 option to ls.


The following examples assume that Alluxio source code exists in the ${ALLUXIO_HOME} directory and an instance of Alluxio is running locally.

Transparent Naming

Create a temporary directory in the local file system that will be used as the under storage to mount:

$ cd /tmp
$ mkdir alluxio-demo
$ touch alluxio-demo/hello

Mount the local directory created into Alluxio namespace and verify it appears in Alluxio:

$ ./bin/alluxio fs mount /demo file:///tmp/alluxio-demo
Mounted file:///tmp/alluxio-demo at /demo
$ ./bin/alluxio fs ls -R /
... # note that the output should show /demo but not /demo/hello

Verify that the metadata for content not created through Alluxio is loaded into Alluxio the first time it is accessed:

$ ./bin/alluxio fs ls /demo/hello
... # should contain /demo/hello

Create a file under the mounted directory and verify the file is not yet persisted to the underlying file system:

$ ./bin/alluxio fs touch /demo/hello2
/demo/hello2 has been created
$ ls /tmp/alluxio-demo

Verify that after manually persisting the file, it is created in the underlying file system with the same name:

$ ./bin/alluxio fs persist /demo/hello2
Found 1 files to persist.
(1/1) Successfully persisted file: /demo/hello2
$ ls /tmp/alluxio-demo
hello hello2

Rename a file in Alluxio and verify the corresponding file is also renamed in the underlying file system:

$ ./bin/alluxio fs mv /demo/hello2 /demo/world
Renamed /demo/hello2 to /demo/world
$ ls /tmp/alluxio-demo
hello world

Delete a file in Alluxio and verify the file is also deleted in the underlying file system:

$ ./bin/alluxio fs rm /demo/world
/demo/world has been removed
$ ls /tmp/alluxio-demo

Unmount the mounted directory and verify that the directory is removed from the Alluxio namespace, but is still preserved in the underlying file system:

$ ./bin/alluxio fs unmount /demo
Unmounted /demo
$ ./bin/alluxio fs ls -R /
... # should not contain /demo
$ ls /tmp/alluxio-demo

Metadata Active Sync for HDFS

In version 2.0, we introduced a new feature for maintaining synchronization between Alluxio space and the UFS when the UFS is HDFS. The feature, called active sync, listens for HDFS events and periodically synchronizes the metadata between the UFS and Alluxio namespace as a background task on the master. Because active sync feature depends on HDFS events, this feature is only available when the UFS HDFS versions is later than 2.6.1. You might need to change the value for alluxio.underfs.version in your configuration file. Please refer to HDFS Under Store for a list of supported Hdfs versions.

To enable active sync on a directory, issue the following Alluxio command.

$ ./bin/alluxio fs mkdir /syncdir
$ ./bin/alluxio fs startSync /syncdir

You can control the active sync interval by changing the alluxio.master.ufs.active.sync.interval option, the default is 30 seconds.

To disable active sync on a directory, issue the following Alluxio command.

$ ./bin/alluxio fs stopSync /syncdir

You can also examine which directories are currently under active sync.

$ ./bin/alluxio fs getSyncPathList

Quiet period for Active Sync

Active sync also tries to avoid syncing when the target directory is heavily used. It tries to look for a quiet period in UFS activity to start syncing between the UFS and the Alluxio space, to avoid overloading the UFS when it is busy. There are two configuration options that control this behavior.

alluxio.master.ufs.active.sync.max.activities is the maximum number of activities in the UFS directory. Activity is a heuristic based on the exponential moving average of number of events in a directory. For example, if a directory had 100, 10, 1 event in the past three intervals. Its activity would be 100/10*10 + 10/10 + 1 = 3 alluxio.master.ufs.active.sync.max.age is the maximum number of intervals we will wait before synchronizing the UFS and the Alluxio space.

The system guarantees that we will start syncing a directory if it is “quiet”, or it has not been synced for a long period (period longer than the max age).

For example, the following setting


means that every 30 seconds, the system will count the number of events in the directory and calculate its activity. If the activity is less than 100, it will be considered a quiet period, and syncing will start for that directory. If the activity is greater than 100, and it has not synced for the last 5 intervals, or 5 * 30 = 150 seconds, it will start syncing the directory. It will not perform active sync if the activity is greater than 100 and it has synced at least once in the last 5 intervals.

Unified Namespace

This example will mount multiple under storages of different types to showcase the unified file system namespace abstraction. This example will use two S3 buckets owned by different AWS accounts and a HDFS service.

Mount the first S3 bucket into Alluxio using its corresponding credentials <accessKeyId1> and <secretKey1> :

./bin/alluxio fs mkdir /mnt
./bin/alluxio fs mount --option aws.accessKeyId=<accessKeyId1> --option aws.secretKey=<secretKey1>  /mnt/s3bucket1 s3://data-bucket1/

Mount the second S3 bucket into Alluxio using its corresponding credentials <accessKeyId2> and <secretKey2>:

$ ./bin/alluxio fs mount \
  --option aws.accessKeyId=<accessKeyId2> \
  --option aws.secretKey=<secretKey2> \
  /mnt/s3bucket2 s3://data-bucket2/

Mount the HDFS storage into Alluxio:

$ ./bin/alluxio fs mount /mnt/hdfs hdfs://<NAMENODE>:<PORT>/

All three directories are all contained in one space in Alluxio:

$ ./bin/alluxio fs ls -R /
... # should contain /mnt/s3bucket1, /mnt/s3bucket2, /mnt/hdfs