A database should not lose any committed data on `kill -9` in the default configuration. This is why PostgreSQL waits on fsync on the write-ahead log before completing a commit. This can be disabled with synchronous_commit off, in which case you will indeed lose data on a crash.
I said it already elsewhere in this thread, but either the writer has other persistent storage, then it should just keep the data and retry later and nothing will be lost, or the writer doesn't have other storage, then you will also lose uncommitted data with a normal shutdown, as you can not even try to commit any data while a data base is down.
