All Your GUCs in a Row: io_max_concurrency

PostgreSQL 18 introduces io_max_concurrency, a per-process hard ceiling on in-flight I/O operations, part of the new asynchronous I/O subsystem. Unlike effective_io_concurrency, which is only a request, this parameter is a grant enforced at the postmaster level, requiring a restart to change. Its default of -1 auto-calculates a value from shared_buffers divided across all possible processes, floored at 1 and capped at 64. Overprovisioned max_connections combined with small shared_buffers can silently resolve this value down to single digits, throttling I/O concurrency with no warning in the logs. Anyone raising effective_io_concurrency for fast storage (NVMe, striped cloud volumes) must also raise io_max_concurrency or the extra concurrency requested is never actually used. The pg_aios view lets administrators check actual in-flight I/O against the ceiling to decide whether raising it is warranted.

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