Many applications find sync writes to replica memory on other rack(s) to be a good substitute for sync to disk. The disk sync can be batched every second or so

Many applications find sync writes to replica memory on other rack(s) to be a good substitute for sync to disk. The disk sync can be batched every second or so for higher throughput and your application is safe. Where this really plays out well is when the disk goes slow for a minute (cloud-style) and the database stays fast.