Increasing Magic Pocket write throughput by removing our SSD cache disks

This post details the removal of SSD cache disks from Dropbox's Magic Pocket storage system. Previously, live writes were committed to SSDs as a write-back cache before being asynchronously flushed to SMR disks. This architecture created a bottleneck, limiting host write throughput to the maximum write throughput of the single SSD. The team re-architected the system to write directly to SMR disks, storing block metadata and raw block data inline within a new extent format. This change increased write throughput by 15-20%, improved reliability by removing a single point of failure (SSD failures), and reduced operational overhead and costs associated with sourcing and maintaining SSDs. While p95 latencies for disk writes increased slightly, overall end-to-end write latency saw a minimal increase, and read latency for recently written blocks was impacted but not significantly at the p99 level.