A well-known performance booster in MySQL is to set sync_binlog to 0. However, this configuration alone comes with serious consequences on consistency and on durability (the C and D of ACID); I explore those in this series. In this post, I give some background on the sync_binlog parameter and I explain part of the problem with setting it to 0 (or to a value different from 1). The other problems — including the behaviour with GTIDs (I am limiting the current scope to legacy file + position replication) — and some solutions are the subject of the upcoming posts.