Why millions of small files per directory collapse a filesystem, and how a SeaweedFS-style volume layout fixes it: append-only superblocks, 8-byte-aligned needles with CRC32C frames, hole-punching deletion, and crash recovery by truncation. Teaches: the real cost of per-blob files, why alignment beats variable-length framing, and how to make crash recovery O(1).
The R4 POSIX core: inodes as content-addressed manifests, files as blobs, atomic rename, hardlinks, permission enforcement — a filesystem on momo.
Verify-on-read re-hashes the full object on every local read (~8x slower than write). A ReadVerifier seam skips re-verification for blobs verified this process, while scrub still catches bitrot.
S3 integrity checksums moved from a surface-level adapter into the storage/ingest core via a protocol-agnostic ChecksumProvider seam — every write path verified, replicas re-verify on receive.
The [storage] backend seam swaps blob storage (local, nfs, s3, raw) behind one Store interface, keeping CAS metadata local.
Identify every object by its SHA-256 content hash — server-side dedup, verify-on-read, and a path that later mounted a filesystem. Teaches: content addressing principles, single write chokepoint, Bolt/Sentinel patterns.