We are a messaging application, so every chat message is stored in DynamoDB in an ordered stream. We frequently fetch slices of a stream, so the granularity of storage is a single message.
S3 does not do well (cost wise) with billions of files. Provisioning 1000 Write IOPS on Dynamo costs around $100/month. That's 2.5B writes. On S3 that's going to be on the order of $10k. Similarly 1000 Read IOPS on Dynamo is ~$20/month vs 2.5B reads on s3 costing $1000. Storage is cheaper on S3 by about 10x, but per TB that comes out to $250 vs $25, which is hardly a dominating cost factor.
S3 does not do well (cost wise) with billions of files. Provisioning 1000 Write IOPS on Dynamo costs around $100/month. That's 2.5B writes. On S3 that's going to be on the order of $10k. Similarly 1000 Read IOPS on Dynamo is ~$20/month vs 2.5B reads on s3 costing $1000. Storage is cheaper on S3 by about 10x, but per TB that comes out to $250 vs $25, which is hardly a dominating cost factor.