S3 Sync version 4 has been released. The latest version supports downloading files that have been moved to the Glacier storage class. Amazon Glacier is a file archiving service. Storing a file in Glacier costs 1/10th the cost of storing it in S3. The drawback of Glacier is that if you want to retrieve a file you need to put in a request and wait up to 5 hours before the file becomes available.
You can use a S3 lifecycle policy to move files from S3 to Glacier. Create a S3 lifecycle policy through Amazon’s AWS Management Console web application. When creating a lifecycle policy you can set the number of days before a file on S3 is moved to Glacier. Amazon does the work of monitoring this policy and moving files when required.
When a file is moved to Glacier it can no longer be downloaded through the Internet. It needs to be restored before it can be retrieved. The latest S3 Sync can detect if your files are on the Glacier storage class and it makes a request to restore them if required. The application waits until the restore is complete then downloads the files.
Glacier is great for backing up large files that you do not need immediate access to. For example backing up videos on S3 is expensive because the files are large. You usually only need to retrieve the files if your local copy is lost. When you store the files on Glacier the cost is low and the files can be available within few hours.
Try the latest version of S3 Sync and see how easy it is to restore files from Glacier.