Amazon S3 Files and Zero FS both present POSIX filesystems backed by S3, but their storage layouts reflect opposite design goals. S3 Files, built on EFS, maintains a one-file-to-one-object mapping so that images/cat.jpg on the mount corresponds to the same key in the bucket. Changes flow bidirectionally after a 60-second write-quiescence window; S3-side edits win conflicts, moving the filesystem version to lost+found. Zero FS instead packs compressed, encrypted file-data frames into immutable segment objects and stores metadata in an LSM tree. The bucket holds ciphertext only; mounting or recovery requires Zero FS and its encryption password, and no S3 API call can fetch a mounted pathname.

Write paths diverge sharply. S3 Files writes to AWS-managed high-performance storage, durably, then asynchronously exports to S3. Over 9P, Zero FS fsync uploads data segments and flushes LSM metadata to object storage before returning success. For cold reads, S3 Files imports directory metadata and files below a 128 KiB threshold into the high-performance tier — AWS notes a 1,000-object listing may take several seconds — while larger reads route directly to S3. Zero FS populates a local RAM and disk cache from reads and sealed segments; a cold miss resolves extents via the LSM tree and coalesces adjacent frames into ranged GETs, avoiding full-file or full-directory imports.

Cost models differ as much as architectures. S3 Files charges for high-performance storage at $0.30 per GB-month, plus $0.03 per GB reads and $0.06 per GB writes to that tier, atop standard S3 fees. In an illustrative 10,000 GiB scenario with small resident reads, S3 Files tallied $3,530 monthly versus Zero FS estimates of $115–$230 plus infrastructure, assuming 2:1 compression. Zero FS requires one or two servers with ~2 GB RAM beyond configured cache. S3 Files may cost less for large-file streaming; the gap widens when data is written through or imported into the high-performance tier.