Amazon S3 Files

• NFS close-to-open consistency vs S3 atomic-PUT strong consistency is bridged by 60-second write aggregation. Writes are visible via NFS immediately but don't land in the bucket via REST API until the next flush.
• Concurrent NFS + REST modifications resolve via a strict "S3 wins" policy — direct API writes silently overwrite in-flight NFS edits.
• Rename is not free. Under the hood it's copy + delete, so high-churn rename patterns incur real cost. Design around append/close rather than rename-as-commit.
• Metadata-only cache mode is the right call for large sequential workloads (pretraining video, uncompressed text corpora) — full file caching just wastes the EFS tier.
• Locking is advisory only. Mandatory locks aren't supported, and direct S3 REST writes bypass NFS locks entirely. Don't rely on the lock primitive for cross-tenant isolation.
• Glacier / Deep Archive objects don't appear under the mount until restored — surprising failure mode for migration scripts that walk a tiered bucket.
• Object key length still caps at 1,024 bytes, no relief from the file-system layer. Deeply nested paths with long names can hit the ceiling without warning.
• Max 128 mount targets per bucket, no exception path.
• Windows is not supported — FSx for Windows File Server is the AWS-blessed alternative.
• Object keys that aren't valid POSIX filenames silently don't appear under the mount, even though they're listable via the S3 API.

Pricing posture: EFS cache tier $0.30/GB-mo + reads $0.03/GB + writes $0.06/GB + metadata operations $0.005 per 1,000 + standard S3 rates underneath. The economics favor "small hot working set on top of a large cold bucket" — a 1 PB bucket with 1 TB active pays cache rates on the 1 TB only.

Companion change you'll see at the same time: SSE-C is disabled by default on new buckets after April 6, 2026 — AWS steering customers to KMS CMKs which carry rotation, audit, and recovery semantics SSE-C never had. Existing SSE-C buckets keep working.