S3 Is Not a Filesystem (But Now There’s One In Front of It)

AWS launched S3 Files, a new feature that lets you mount an S3 bucket as a shared NFS 4.1/4.2 filesystem on EC2, Lambda, EKS, and ECS. Unlike previous FUSE-based hacks (s3fs-fuse, goofys, Mountpoint), it's built on EFS infrastructure with S3 as the durable backing store — each layer does what it's good at, with automatic syncing between them. Pricing mirrors EFS Performance-optimized mode ($0.30/GB-month for hot storage, $0.03/GB reads, $0.06/GB writes), but you only pay the filesystem surcharge on the hot slice you actually touch; large files (128 KB+) stream directly from S3 at no S3 Files charge. Key gotchas include a 32 KB minimum per operation and sync cost multipliers that can triple effective read costs for small files. Compared to EFS, S3 Files wins on large-file reads (free vs. $0.03/GB) and cold storage tiering (free with S3 IT vs. $0.01–$0.03/GB per EFS tier transition), while EFS wins on small-file first-read cost. Best suited for ML training pipelines, agentic AI workloads needing shared storage, and legacy POSIX apps currently running on EFS.