THE PAPER THAT SPLIT THE DATABASE IN FOUR: INSIDE AZURE SQL DATABASE HYPERSCALE, AKA SOCRATES

Summary from the TLDR Data newsletter

Azure SQL once kept four full copies of each database, which capped practical size at around 4 TB. The 2019 Socrates paper separated the log from storage, so durability and availability could scale apart. This change in approach let Hyperscale reach 100 TB, halve storage copies, and restore a 50 TB database in minutes.