Alex Miller writes about database internals and transactional systems, including disk I/O, replication, Raft and a BerkeleyDB clone tutorial.
HN commenters point to it for its disk I/O diagrams, the BerkeleyDB tutorial series and the copy-and-patch write-up.
A B-Tree tutorial series implementing an ABI-compatible BerkeleyDB clone.
Direct links to the articles:
1. https://transactional.blog/building-berkeleydb/page-format
2. https://transactional.blog/building-berkeleydb/entry-format
3. https://transactional.blog/building-berkeleydb/api-basics
4. https://transactional.blog/building-berkeleydb/point-reads
the accompanying post "How It Works" is worth reading alongside this tutorial
https://transactional.blog/copy-and-patch/
(key terms: abus[e|ing]: 4, force: 3, trick: 1, chance: 1)
Take a look at Alex Miller's diagrams for what function calls are actually doing on various systems.
https://transactional.blog/how-to-learn/disk-io
https://transactional.blog/talk/enough-with-all-the-raft