I've never implemented database transactions, so this might be a little hand wavy. A database transaction promises that all changes happen or no changes happen

I've never implemented database transactions, so this might be a little hand wavy. A database transaction promises that all changes happen or no changes happen, i.e., that you don't get some of the changes. The easiest way to implement that is to write what you're going to do to a log, and then once you know that the log is on disk, you make the change in the actual data file. Two phase commit. But what if I could loosen that some? It isn't that important when the log file is actually written; only that it will hit disk before the changes to the data file. If I write to the log, tell the operating system "after you get around to putting that on disk, make these changes to this other file," I can promise that the transaction will be atomic. I can't tell you if the transaction will actually be committed, but I know there won't be a partial commit. If the system crashes before the log gets updated, the transaction doesn't commit. But I don't have to wait around to find out when the file gets written out. I just need to know that if anything gets written to the data file, then all my writes to the log file are safely on disk.