It's somewhat related to the number of data drives, but as you point out it's a bit more complicated in practice - it's not just the number of drives. For examp

It's somewhat related to the number of data drives, but as you point out it's a bit more complicated in practice - it's not just the number of drives. For example due to parallelism built into SSDs (each SSD is actually a stack of smaller drives). For rotational devices it's complicated by TCQ/NCQ, optimizations that require a queue of commands for each device to actually work. On modern SSD, values 5-8 per device are definitely on the low side. See for example this thread from 2014, on (then released S3500 SSD from Intel): https://www.postgresql.org/message-id/CAHyXU0yiVvfQAnR9cyH%3DHWh1WbLRsioe%3DmzRJTHwtr%3D2azsTdQ%40mail.gmail.com https://www.postgresql.org/message-id/CAHyXU0yiVvfQAnR9cyH%3... That shows the performance increases up to effective_io_concurrency=256 (of course, in reality there may be multiple queries running on the system, sharing the I/O paths).