Unlike S3, Amazon's Glacier service does not offer immediate download (restore). When restoring from Glacier, Arq must first request that the objects be made downloadable. This takes approximately 4 hours. Once the objects are downloadable, Arq downloads them.
When you click “Restore”, Arq begins requesting objects, and continues until it has requested the amount of data that would take 4 hours to download at the rate you’ve chosen. After 4 hours have elapsed, it begins requesting another 4 hours’ worth of objects, and simultaneously begins downloading objects that are becoming available. It continues this pattern until all the files have been downloaded.
(The following pricing numbers are based on the Amazon "US East" region.)
Calculating pricing for downloading your data from Glacier can be confusing.
Restoring a file from Glacier is a 3-step process. First you issue a Glacier restore request for an object. Then you wait approximately 4 hours for the object to become available for download. Then you download it.
Requesting that an object be made available for download costs $.05/1000 requests, and data transfer out of Glacier is $.12/GB with the first 1GB free each month.
You can restore up to 5% of your Glacier data for free each month, prorated daily. For example, if you’ve backed up 100GB of files to Glacier, you can restore 5GB for free each month — 160MB each day.
If you exceed the 5% in a month, Amazon charges a data restore fee. Amazon’s description of the data restore fee is complex. Put more simply, the data restore fee is equal to the total size (in GB) of the object(s) requested, multiplied by $7.20, divided by 4 hours, minus the prorated 5% free tier.
Also, the data restore fee is only incurred once for the entire billing month. If you request objects from Glacier and incur a data restore fee of, say $5, you could continue requesting objects from Glacier for the rest of the month at that rate (or slower), and the charge on your bill for that month will be $5. But if, during some other hour in that month, you request objects at a rate that equals a data restore fee of, say, $6, the charge at the end of the month will be $6.
If you requested all 100GB of your Glacier data all at once, the data restore fee could be very high. But in practice that isn’t realistic. For one thing, you probably can’t download 100GB of data very quickly. A 10 megabit/second ISP connection would allow downloading of about 3.6GB/hour; 100GB would take 27 hours to download, so there’s no need to ask Glacier to make all 100GB available in 4 hours.
The best way to download from Glacier is to request only what you can download in the next 4 hours, and repeat every 4 hours as necessary while you download objects as they become available for download.
Arq manages the Glacier retrieval cost for you. When you restore your files from Arq Glacier backups, you first select a transfer rate, and Arq’s Amazon Glacier calculator calculates the data restore fee (labeled “peak hourly request fee”) for you:
If you change the download rate, Arq updates the cost estimates. In this example we’ve changed from 661 KB/sec to 341 KB/sec, and the data restore fee is cut in half:
Choose the transfer rate that makes sense for you, based on your available download speed and how quickly you need the data.