Event sourcing append-only store

Design an append-only event store for an event-sourced system: events are immutable facts grouped into per-aggregate streams, writers append to a stream with an expected-version for optimistic concurrency, and readers replay a stream from the beginning to rebuild state or subscribe to a global feed of new events. Expect millions of streams, ~100K appends/sec, and a guarantee that appends to one stream are strictly ordered. How do you store and serve this?