Dear developers: I have some suggestions on PostgreSQL's IO behavior. I am not sure my opinions are right. Many thanks for your time!
It is documented that : wal_sync_method (enum): The open_* options also use O_DIRECT if available. Shall PostgreSQL consider making O_DIRECT configurable? In MySQL, innodb_flush_method can change if use O_DIRECT or not. I made some benchmarking on O_DIRECT of MySQL, and I find that sometimes, using O_DIRECT may hurt performance. https://ibb.co/BVWnwnh [https://i.ibb.co/1fyzbzC/003.png]<https://ibb.co/BVWnwnh> 003<https://ibb.co/BVWnwnh> Image 003 hosted in ImgBB ibb.co Comparing the blue (buffered IO) and green (direct IO) lines, I find buffered IO is faster. So I think having such parameter in PostgreSQL is reasonable. On the other hand, wal_sync_method only controls how WAL is written to devices, while for data file I notice that PostgreSQL uses sync_file_range(2). So shall we also make it configurable? I also find that in some systems, open with O_SYNC is much more faster than write + fsync : https://ibb.co/f1VsCC1 [https://i.ibb.co/gjbxDDj/Picture1.png]<https://ibb.co/f1VsCC1> Picture1<https://ibb.co/f1VsCC1> Image Picture1 hosted in ImgBB ibb.co Furthermore, the results above are also related to IO API supported by OS. MySQL support synchronized IO and Linux libaio. It seems that PostgreSQL only supports synchronized IO, so shall we support more IO engines? like io_uring which is very popular in recent years. Thanks & best, Haochen
