Yesterday, Amazon announced a great new feature for S3: cross-region replication. It is yet another in a long list of improvements that increase the value and power of AWS.
However, before you jump on the bandwagon, there is an important safety tip: Check the pricing carefully! AWS charges the normal S3 data transfer cost when S3 migrates objects from one region to another. Storing 1 GB in S3 costs $.03/month. Transferring 1 GB from one S3 region to another costs $.02/GB. In other words, it costs 68% as much to transfer data to another region once as it does to store it for an entire month.
If you are making one remote-region backup of a stable object that you are going to keep for a while, that can be a perfectly reasonable price to pay. However, if you are thinking, "Hey, now we can store our nightly backups in the local region and in a remote region for only twice the cost, with no extra effort!," think again. Transferring an object from one S3 region to another every night will increase your total monthly cost for those objects by twenty times, because 30*68% = 20.4. That cost might still be fine for you, but you should certainly be aware of it. Caveat emptor. :-)
That said, we are already evaluating using S3 cross-region replication for several use cases and believe this is once again a great demonstration of AWS innovation.