Inside the Magic Pocket

This post provides a high-level architectural overview of Magic Pocket, Dropbox's immutable block storage system for file content. It details the system's design requirements: immutable block storage (up to 4MB chunks), workload characteristics (high temporal locality, low-latency reads), durability (effectively infinite theoretical durability via erasure coding and multi-region replication), scale (transition from double-digit petabytes to multi-exabytes), and simplicity (avoiding complex consensus). The data model is explained: blocks (identified by SHA-256 hash) are aggregated into 1GB buckets, which are then stored in volumes across replicated or erasure-coded physical nodes. The multi-zone architecture is described, with frontends, a sharded MySQL Block Index service, cross-zone replication daemons, and cells (logical storage clusters of ~50PB).