Waiting for Postgres 18: Accelerating Disk Reads with Asynchronous I/O

Summary

With the Postgres 18 Beta 1 release this week a multi-year effort, and significant architectural shift in Postgres is taking shape: Asynchronous I/O (AIO). These capabilities are still under active development, but they represent a fundamental change in how Postgres handles I/O, offering the potential for significant performance gains, particularly in cloud environments where latency is often the bottleneck. While some features may still be adjusted or dropped during the beta period before the final release, now is the best time to test and validate how Postgres 18 performs in practice. In Postgres 18 AIO is limited to read operations; writes remain synchronous, though support may expand in future versions. In this post, we explain what asynchronous I/O is, how it works in Postgres 18, and what it means for performance optimization. Why asynchronous I/O matters Postgres has historically operated under a synchronous I/O model, meaning every read request is a blocking system call. The database must pause and wait for the operating system to return the data before continuing. This design introduces unnecessary waits on I/O, especially in cloud environments where storage is often network-attached (e.g. Amazon EBS) and I/O can have over 1ms of latency. In a simplified model, we can illustrate the difference like this, ignoring any prefetching/batching the Linux kernel might do: You can picture synchronous I/O like an imaginary librarian who retrieves one book at a time, returning before fetching the next. This inefficiency compounds as the number of physical reads for a logical operation increases. Asynchronous I/O eliminates that bottleneck by allowing programs to issue multiple read requests concurrently, without waiting for prior reads to return. In an async program flow, I/O requests are scheduled to be read into a memory location and the program waits for completion of those reads, instead of issuing each read individually. How Postgres 17’s read streams paved the w...