Those VM tunables are about dirty OS cache, not dirty drive cache. If you fsync() a file on Linux it will be pushed to the drive and (if the drive does not have battery/capacitor-backed cache) flushed from drive cache to stable storage. If you don't fsync() then AIUI all bets are off, but in practice the drive will eventually get around to flushing your data anyway. The OS has one timeout for cache flushes and the drive should have another one, one would hope.