AWS S3 Files is a managed POSIX/NFS file system backed by S3 storage, offering up to 13x lower storage costs than EFS ($0.023/GB vs $0.30/GB) for large-file workloads. The post covers how S3 Files works (file system, mount target, access point), the full cost comparison including edge cases where S3 Files is actually more expensive (small files under 1 MiB), the critical 60-second write-back delay that breaks coordination patterns, POSIX limitations (no hard links, no cross-client file locking, non-atomic directory renames), and a step-by-step AWS SAM setup for Lambda including VPC networking, service role, and file system provisioning. A decision table helps choose between S3 Files and EFS based on file size, consistency needs, and concurrency requirements.
Table of contents
Where the 13x Comes FromWhy EFS Bills Shock YouHow S3 Files Actually WorksThe Cost MathThe 60-Second Write-Back DelayPOSIX LimitsArchitecture Deep DiveSetting It Up with LambdaS3 Files vs. EFS — When to Use WhichWrapping UpSort: