The aws s3 transfer commands, which include the cp, sync, mv, and rm commands, have additional configuration values you can use to control S3 transfers. This topic guide discusses these parameters as well as best practices and guidelines for setting these values.

Before discussing the specifics of these values, note that these values are entirely optional. You should be able to use the aws s3 transfer commands without having to configure any of these values. These configuration values are provided in the case where you need to modify one of these values, either for performance reasons or to account for the specific environment where these aws s3 commands are being run.

These are the configuration values you can set for S3:

• max_concurrent_requests - The maximum number of concurrent requests.
• max_queue_size - The maximum number of tasks in the task queue.
• multipart_threshold - The size threshold the CLI uses for multipart transfers of individual files.
• multipart_chunksize - When using multipart transfers, this is the chunk size that the CLI uses for multipart transfers of individual files.

These values must be set under the top level s3 key in the AWS Config File, which has a default location of ~/.aws/config. Below is an example configuration:

[profile development]
aws_access_key_id=foo
aws_secret_access_key=bar
s3 =
  max_concurrent_requests = 20
  max_queue_size = 10000
  multipart_threshold = 64MB
  multipart_chunksize = 16MB

Note that all the S3 configuration values are indented and nested under the top level s3 key.

You can also set these values programatically using the aws configure set command. For example, to set the above values for the default profile, you could instead run these commands:

$ aws configure set default.s3.max_concurrent_requests 20
$ aws configure set default.s3.max_queue_size 10000
$ aws configure set default.s3.multipart_threshold 64MB
$ aws configure set default.s3.multipart_chunksize 16MB

Default - 10

The aws s3 transfer commands are multithreaded. At any given time, multiple requests to Amazon S3 are in flight. For example, if you are uploading a directory via aws s3 cp localdir s3://bucket/ --recursive, the AWS CLI could be uploading the local files localdir/file1, localdir/file2, and localdir/file3 in parallel. The max_concurrent_requests specifies the maximum number of transfer commands that are allowed at any given time.

You may need to change this value for a few reasons:

• Decreasing this value - On some environments, the default of 10 concurrent requests can overwhelm a system. This may cause connection timeouts or slow the responsiveness of the system. Lowering this value will make the S3 transfer commands less resource intensive. The tradeoff is that S3 transfers may take longer to complete.
• Increasing this value - In some scenarios, you may want the S3 transfers to complete as quickly as possible, using as much network bandwidth as necessary. In this scenario, the default number of concurrent requests may not be sufficient to utilize all the network bandwidth available. Increasing this value may improve the time it takes to complete an S3 transfer.

Default - 1000

The AWS CLI internally uses a producer consumer model, where we queue up S3 tasks that are then executed by consumers, which in this case utilize a bound thread pool, controlled by max_concurrent_requests. A task generally maps to a single S3 operation. For example, as task could be a PutObjectTask, or a GetObjectTask, or an UploadPartTask. The enqueuing rate can be much faster than the rate at which consumers are executing tasks. To avoid unbounded growth, the task queue size is capped to a specific size. This configuration value changes the value of that maximum number.

You generally will not need to change this value. This value also corresponds to the number of tasks we are aware of that need to be executed. This means that by default we can only see 1000 tasks ahead. Until the S3 command knows the total number of tasks executed, the progress line will show a total of .... Increasing this value means that we will be able to more quickly know the total number of tasks needed, assuming that the enqueuing rate is quicker than the rate of task consumption. The tradeoff is that a larger max queue size will require more memory.

Default - 8MB

When uploading, downloading, or copying a file, the S3 commands will switch to multipart operations if the file reaches a given size threshold. The multipart_threshold controls this value. You can specify this value in one of two ways:

• The file size in bytes. For example, 1048576.
• The file size with a size suffix. You can use KB, MB, GB, TB. For example: 10MB, 1GB. Note that S3 imposes constraints on valid values that can be used for multipart operations.

Default - 8MB

Minimum For Uploads - 5MB

Once the S3 commands have decided to use multipart operations, the file is divided into chunks. This configuration option specifies what the chunk size (also referred to as the part size) should be. This value can specified using the same semantics as multipart_threshold, that is either as the number of bytes as an integer, or using a size suffix.