mmap vs pread in a real Go storage engine

When you build a storage engine in Go, sooner or later you need to answer a very plain question: “How should the code read bytes from files?” This sounds too low-level to matter. A database has bigger ideas: partitions, blocks, indexes, filters, compression, compaction, caches, query planning. But all of these ideas end up doing the same simple action many times: “Read N bytes from file F at offset O.”