Amazon AWS S3

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This guide describes the instructions to configure Amazon S3 as Alluxio’s under storage system.


The Alluxio binaries must be on your machine. You can either compile Alluxio, or download the binaries locally.

In preparation for using S3 with Alluxio, create a new bucket or use an existing bucket. You should also note the directory you want to use in that bucket, either by creating a new directory in the bucket, or using an existing one. For the purposes of this guide, the S3 bucket name is called S3_BUCKET, and the directory in that bucket is called S3_DIRECTORY.

Basic Setup

Alluxio unifies access to different storage systems through the unified namespace feature. An S3 location can be either mounted at the root of the Alluxio namespace or at a nested directory.

Root Mount Point

Create conf/ if it does not exist.

$ cp conf/ conf/

Configure Alluxio to use S3 as its under storage system by modifying conf/ Specify an existing S3 bucket and directory as the under storage system by modifying conf/ to include:


Note that if you want to mount the whole s3 bucket, add a trailing slash after the bucket name (e.g. s3://S3_BUCKET/).

Specify the AWS credentials for S3 access by setting s3a.accessKeyId and s3a.secretKey in

s3a.accessKeyId=<S3 ACCESS KEY>
s3a.secretKey=<S3 SECRET KEY>

For other methods of setting AWS credentials, see the credentials section in Advanced Setup.

After these changes, Alluxio should be configured to work with S3 as its under storage system, and you can try Running Alluxio Locally with S3.

Nested Mount

An S3 location can be mounted at a nested directory in the Alluxio namespace to have unified access to multiple under storage systems. Alluxio’s Command Line Interface can be used for this purpose.

$ ./bin/alluxio fs mount \
  --option s3a.accessKeyId=<AWS_ACCESS_KEY_ID> \
  --option s3a.secretKey=<AWS_SECRET_KEY_ID> \
  /mnt/s3 s3://<S3_BUCKET>/<S3_DIRECTORY>

Running Alluxio Locally with S3

Start up Alluxio locally to see that everything works.

$ ./bin/alluxio format
$ ./bin/ local

This should start an Alluxio master and an Alluxio worker. You can see the master UI at http://localhost:19999.

Before running an example program, please make sure the root mount point set in the is a valid path in the ufs. Make sure the user running the example program has write permissions to the alluxio file system.

Run a simple example program:

$ ./bin/alluxio runTests

Visit your S3 directory s3://<S3_BUCKET>/<S3_DIRECTORY> to verify the files and directories created by Alluxio exist. For this test, you should see files named like:


To stop Alluxio, you can run:

$ ./bin/ local

Advanced Setup

Configure S3 Region

Configure S3 region when accessing S3 buckets to improve performance. Otherwise, global S3 bucket access will be enabled which introduces extra requests. S3 region can be set in conf/


Advanced Credentials Setup

You can specify credentials in different ways, from highest to lowest priority:

  1. s3a.accessKeyId and s3a.secretKey specified as mount options
  2. s3a.accessKeyId and s3a.secretKey specified as Java system properties
  3. s3a.accessKeyId and s3a.secretKey in
  4. Environment Variables AWS_ACCESS_KEY_ID or AWS_ACCESS_KEY (either is acceptable) and AWS_SECRET_ACCESS_KEY or AWS_SECRET_KEY (either is acceptable) on the Alluxio servers
  5. Profile file containing credentials at ~/.aws/credentials
  6. AWS Instance profile credentials, if you are using an EC2 instance

When using an AWS Instance profile as the credentials’ provider:

  • Create an IAM Role with access to the mounted bucket
  • Create an Instance profile as a container for the defined IAM Role
  • Launch an EC2 instance using the created profile

Note that the IAM role will need access to both the files in the bucket as well as the bucket itself in order to determine the bucket’s owner. Automatically assigning an owner to the bucket can be avoided by setting the property alluxio.underfs.s3.inherit.acl=false.

See Amazon’s documentation for more details.

Enabling Server Side Encryption

You may encrypt your data stored in S3. The encryption is only valid for data at rest in S3 and will be transferred in decrypted form when read by clients. Note, enabling this will also enable HTTPS to comply with requirements for reading/writing objects.

Enable this feature by configuring conf/



By default, a request directed at the bucket named “mybucket” will be sent to the host name “”. You can enable DNS-Buckets to use path style data access, for example: “” by setting the following configuration:


Accessing S3 through a proxy

To communicate with S3 through a proxy, modify conf/ to include:<PROXY_HOST>

<PROXY_HOST> and <PROXY_PORT> should be replaced by the host and port of your proxy.

Using a non-Amazon service provider

To use an S3 service provider other than “”, modify conf/ to include:


Replace <S3_ENDPOINT> with the hostname and port of your S3 service, e.g., http://localhost:9000. Only use this parameter if you are using a provider other than

Connecting to Oracle Cloud Infrastructure (OCI) object storage

Both the endpoint and region value need to be updated to use non-home region.


All OCI object storage regions need to use PathStyleAccess


Using v2 S3 Signatures

Some S3 service providers only support v2 signatures. For these S3 providers, you can enforce using the v2 signatures by setting the alluxio.underfs.s3.signer.algorithm to S3SignerType.

[Experimental] S3 streaming upload

S3 is an object store and because of this feature, the whole file is sent from client to worker, stored in the local disk temporary directory, and uploaded in the close() method by default.

To enable S3 streaming upload, you need to modify conf/ to include:


The default upload process is safer but has the following issues:

  • Slow upload time. The file has to be sent to Alluxio worker first and then Alluxio worker is responsible for uploading the file to S3. The two processes are sequential.
  • The temporary directory must have the capacity to store the whole file.
  • Slow close(). The execution time of close() method is proportional to the file size and inversely proportional to the bandwidth. That is O(FILE_SIZE/BANDWIDTH). Slow close() is unexpected and has already been a bottleneck in the Alluxio Fuse integration. Alluxio Fuse method which calls close() is asynchronous and thus if we write a big file through Alluxio Fuse to S3, the Fuse write operation will be returned much earlier than the file has been written to S3.

The S3 streaming upload feature addresses the above issues and is based on the S3 low-level multipart upload.

The S3 streaming upload has the following advantages:

  • Shorter upload time. Alluxio worker uploads buffered data while receiving new data. The total upload time will be at least as fast as the default method.
  • Smaller capacity requirement. Our data is buffered and uploaded according to partitions (alluxio.underfs.s3.streaming.upload.partition.size which is 64MB by default). When a partition is successfully uploaded, this partition will be deleted.
  • Faster close(). We begin uploading data when data buffered reaches the partition size instead of uploading the whole file in close().

If a S3 streaming upload is interrupted, there may be intermediate partitions uploaded to S3 and S3 will charge for those data.

To reduce the charges, users can modify conf/ to include:


Intermediate multipart uploads in all non-readonly S3 mount points older than the clean age (configured by alluxio.underfs.s3.intermediate.upload.clean.age) will be cleaned when a leading master starts or a cleanup interval (configured by alluxio.underfs.cleanup.interval) is reached.

Tuning for High Concurrency

When using Alluxio to access S3 with a great number of clients per Alluxio server, these parameters can be tuned so that Alluxio uses a configuration optimized for the S3 backend.

# If the S3 connection is slow, a larger timeout is useful

# If we expect a great number of concurrent metadata operations
# If the total number of metadata + data operations is huge

# For a worker, the number of concurrent writes to S3
# For a master, the number of threads to concurrently rename files within a directory

# Thread-pool size to submit delete and rename operations to S3 on master

Identity and Access Control of S3 Objects

S3 identity and access management is very different from the traditional POSIX permission model. For instance, S3 ACL does not support groups or directory-level settings. Alluxio makes the best effort to inherit permission information including file owner, group and permission mode from S3 ACL information.

Why is 403 Access Denied Error Returned

The S3 credentials set in Alluxio configuration corresponds to an AWS user. If this user does not have the required permissions to access an S3 bucket or object, a 403 permission denied error will be returned.

If you see a 403 error in Alluxio server log when accessing an S3 service, you should double-check

  1. You are using the correct AWS credentials. See credential setup.
  2. Your AWS user has permissions to access the buckets and objects mounted to Alluxio.

Read more AWS troubleshooting guidance for 403 error.

File Owner and Group

Alluxio file system sets the file owner based on the AWS account configured in Alluxio to connect to S3. Since there is no group in S3 ACL, the owner is reused as the group.

By default, Alluxio extracts the display name of this AWS account as the file owner. In case this display name is not available, this AWS user’s canonical user ID will be used. This canonical user ID is typically a long string (like 79a59df900b949e55d96a1e698fbacedfd6e09d98eacf8f8d5218e7cd47ef2be), thus often inconvenient to read and use in practice. Optionally, the property can be used to specify a preset mapping from canonical user IDs to Alluxio usernames, in the format “id1=user1;id2=user2”. For example, edit to include\

This configuration helps Alluxio recognize all objects owned by this AWS account as owned by the user john in Alluxio namespace. To find out the AWS S3 canonical ID of your account, check the console, expand the “Account Identifiers” tab and refer to “Canonical User ID”.

File Permissions

Alluxio checks the S3 bucket read/write ACLs to determine the owner’s permissions to an Alluxio file. For example, if the AWS user has read-only access to the mounted bucket, the mounted directory and files will be set with 0500 permissions. If the AWS user has full access to the underlying bucket, the mounted directory and files will be set with 0700 permissions. Note that Alluxio only inherits bucket-level ACLs when determining file system permissions for a mount point, and ignores the ACLs that are set to individual objects.

With 0500 or 0700 as the default file permissions, Alluxio file system users other than the file owner can not access the files under the mount point. This may create problems for different users to read mounted data. For example, a Presto job running under user presto may encounter an error like Query failed: Failed to list directory, when accessing a mount point owned by the user john with permission bits 0700. In Alluxio master log (master.log), one can find errors like

Error=alluxio.exception.AccessControlException: Permission denied:
user=presto, access=--x, path=/mnt/s3/myobject: failed at s3, inode owner=john, inode group=john, inode mode=rwx------

This is because the mounted directory has permission 0700 and thus application user presto is not able to access this file. To share the S3 mount point with other users in Alluxio namespace, one either can choose of:

  1. (Option1): Set alluxio.master.mount.table.root.shared=true in for root mount or pass --shared on fs mount command; This gives all users rwx permission.
  2. (Option2): Set alluxio.underfs.s3.inherit.acl=false and alluxio.underfs.s3.default.mode to a new default value other than 0700 that can enable other users to access.

Changing Permissions

chown, chgrp, and chmod of Alluxio directories and files do NOT propagate to the underlying S3 buckets nor objects.


Enabling AWS-SDK Debug Level

If issues are encountered when running against your S3 backend, enable additional logging to track HTTP traffic. Modify conf/ to add the following properties:

See Amazon’s documentation for more details.

Prevent Creating Zero-byte Files

Alluxio creates zero-byte files in S3 as a performance optimization. For a bucket mounted with read-write access, zero-byte file creation (S3 PUT operation) is not restricted to write operations using Alluxio but also occurs when listing contents of the under storage. To disable the PUT operation, mount the bucket with the --readonly flag or set alluxio.master.mount.table.root.readonly=true for root mount.