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Efficiently transfer large amounts of data to S3

Project description

S3AM, pronounced \ˈskrēm\, is a fast, parallel, streaming transfer utility for S3. Objects in S3 can be streamed from any URL for which the locally installed libcurl and the remote server support byte range requests, for example file://, ftp:// (many servers) and http:// (some servers). Additionally, objects in S3 can be downloaded to the local file system. Both uploads and downloads can be resumed after interruptions.

S3AM is intended to be used with large files, has been tested with 300GB files but imposes no inherent limit on the maximum file size. While it can be used to transfer small files, you may find that it performs worse than other utilities if the file size is below, say, 5MB.

S3AM supports encrypting the uploaded files with SSE-C.

S3AM can also copy objects between buckets and within a bucket, and do so without actually transferring any data between client and server. It supports the use of separate SSE-C encryption keys for source and destination of the copy operation so it can be used to efficiently re-encrypt an object.

S3AM uses the PyCurl bindings for libcurl and Python’s multiprocessing module to work around lock contention in the Python interpreter and to avoid potential thread-safety issues with libcurl.


Python 2.7.x, libcurl and pip.

On Ubuntu the dependencies can be installed with

sudo apt-get install python-dev gcc make libcurl4-openssl-dev libssl-dev


It is recommended that you install S3AM into a virtualenv:

virtualenv ~/s3am && source ~/s3am/bin/activate
pip install s3am

If you would like to install the latest unstable release, you may want to run pip install --pre s3am instead.

If you get libcurl link-time ssl backend (nss) is different from compile-time ssl backend the required fix is to prefix the pip installation command with PYCURL_SSL_LIBRARY=nss, for example PYCURL_SSL_LIBRARY=nss pip install --pre s3am.

Optionally, add a symbolic link to the s3am command such that you don’t need to activate the virtualenv before using it:

mkdir -p ~/bin
ln -s ~/s3am/bin/s3am ~/bin

Then append ~/bin to your PATH environment variable. I would tell you explicitly how to modify PATH but unfortunately this depends on various factors, e.g. which shell you are using and which operating system. On OS X, for example, you need to edit ~/.profile and append

export PATH="$HOME/bin:$PATH"


Obtain an access and secret key for AWS. Create ~/.aws/credentials with the following contents:


Multiple profiles with credentials for different AWS accounts can coexist in ~/.aws/credentials:



If you specify multiple profiles in ~/.aws/credentials you can choose an active profile by setting the AWS_PROFILE environment variable:

export AWS_PROFILE=foo


Run with --help to display usage information:

s3am --help

To upload a file from an FTP server to S3:

s3am upload \ \

Copy the resulting object to another bucket:

s3am upload \
     s3://foo-bucket/ERR001268.filt.fastq.gz \

Download the copy to the local file system:

s3am download \
     s3://other-bucket/ERR001268.filt.fastq.gz \

Note how all of the above examples omit the file name from the destination. If the destination ends in a / character, the last path component (aka the ‘file name’) of the source URL will be appended.

If an upload was interrupted, it can be resumed by running the command again with the --resume option. To cancel an unfinished upload, run s3am cancel. Be aware that incomplete multipart uploads do incur storage fees.


If you get error: [Errno 104] Connection reset by peer you may be running S3AM with too many upload slots or with too low a part size. Note that by default, S3AM uses a conservatively small part size but allocates one upload slot per core. For example, running s3am on a 32-core EC2 instance and using the default part size of 5 MiB can result in more than 100 requests per second, which will trigger a request rate limiter on the S3 side that could lead to this particular error. Consider passing either --part-size=256MB or --upload-slots=8. The former is recommended as the latter will negatively impact your throughput.


By default S3AM concurrently transfers one part per core. This is a very conservative setting. Since S3AM is mostly IO-bound you should significantly oversubscribe cores, probably by a factor of at least 10. On a machine with 8 cores, for example, you should run S3AM with --download-slots 40 --upload-slots 40.

If you run S3AM on EC2, you will likely have more bandwidth to S3 than from the source server. In this case it might help to have more download than upload slots.

The default part size of 5MB is also very conservative. If the source has a high latency, you will want to increase that as it might take a while for the TCP window to grow to an optimal size. If the source is ftp:// there will be significantly more round-trips before the actual transfer starts than with http:// or http://. In either case you should probably increase the part size to at least 50MB.


Clone the repository, create a virtualenv, activate it and run make develop:

git clone
cd s3am
virtualenv venv
make develop


With SSE-C, the S3 server performs the actual encryption but the client provides the encryption key. This is more secure than plain SSE because with SSE-C the secret encryption key is not persisted on the server, it only exists there in memory for the duration of a request and is discarded afterwards. SSE-C also lets you make a bucket public and control access via the distribution of encryption keys.


You can enable resumption and keep trying a few times:

for i in 1 2 3; do s3am upload --resume $src $dst && break; done
s3am cancel $dst

There are situations after which resumption is futile and care must be taken not to get into an infinite loop that would likely cost an infinite amount of money. S3AM exits with status code 2 on obvious user errors but there may be other failures like auth problems where user intervention is required. There is no reliable way to classify errors into resumable and non-resumable ones so S3AM doesn’t even try. Running s3am cancel is a best effort to avoid leaving unfinished uploads. If s3am upload was successful for a given object, running s3am cancel on that object does nothing.

Alternatively, you can force S3AM to eradicate previous, unsuccessful attempts, creating a clean slate and preventing them from corrupting the current attempt. This comes at the expense of wasting resources by discarding the progress made in those previous attempts:

for i in 1 2 3; s3am upload --force $src $dst && break; done
s3am cancel $dst

The –force and –resume options are mutually exclusive, but both provide a certain degree of idempotence. While --resume refuses to function if it detects multiple unfinished uploads for a given S3 object, --force is not so easily dissuaded. Hence the name.

In a Toil script I would either use the --resume option with a hand-coded loop or the --force option while relying on Toil’s built-in job retry mechanism.


S3AM doesn’t support non-US buckets yet. See issue #12.

S3AM uses a buffer per upload and download slot. The buffer will hold an entire part. This means that the lower bound of S3AM’s memory footprint is (download_slots + upload_slots) * part_size. The buffer is needed because S3 doesn’t support chunked transfer coding.

S3AM does not implement back-to-back checksumming. An MD5 is computed for every part uploaded to S3 but there is no code in place to compare the MD5 with the source side. I think S3 exposes the MD5 of all part MD5’s concatenated. So if we could get libcurl and the sending server to support the Content-MD5 HTTP header we could use that. But that would not be as strong a guarantee as verifying the MD5 over the file in its entirety.

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