[$] Measuring and improving buffered I/O

There are two types of file I/O on Linux, buffered I/O, which goes throughthe page cache, and direct I/O, which goes directly to the storage device.The performance of buffered I/O was reported to be a lot worse than directI/O, especially for one specific test, in Luis Chamberlain's topicproposal for a session at the 2024 Linux Storage,Filesystem, Memory Management, and BPF Summit.The proposal resulted in a lengthy mailing-list discussion, which also came up in Paul McKenney's RCU session the nextday; Chamberlain led a combined storage and filesystem session to discuss those results with aneye toward improving buffered I/O performance.