POSIX allows you never to write data to disk at all. That will make your file system very fast. After all you can have a POSIX-compliant file system that operates off of ramdisk quite easily.
POSIX file system access is designed to describe the interface layer between userland and the file system. It leaves the actual integration between the file system and the hardware, as well as the internals to the file system itself is left up to the developer of the OS.
It is like if you discovered all of a sudden a network service provided by a Apache-based web app uses SSL badly so that all usernames and passwords are transmitted over the Web in plain text... then you complain about it and the developer says back to you that his application's behavior is allowed by TCP/HTTP/SSL and that you should be changing your password with each usage, like people who use his app correctly do. Then he emails you some documentation from a security expert that says you should change your password frequently and that many other protocols like telnet or ftp send your username and password over the network in plain text.
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