How a database keeps concurrent readers and writers out of each other's way, promises a commit and then survives a power cut, and finds one row among hundreds of millions in a handful of page reads — and how all of it is priced, underneath, by the disk's block size and random IOPS.
The set: mvcc for separating readers from writers, isolation-levels for the contract that separation is sold at, wal for durability, b-tree for finding rows, io-block-size-and-iops for the price list everything above is negotiated against.