All Your GUCs in a Row: io_combine_limit and io_max_combine_limit

PostgreSQL 17 introduced read streams that merge adjacent disk blocks into larger I/O requests via io_combine_limit (default 128kB, 16 blocks). PostgreSQL 18 raised the maximum to 128 blocks (1MB) on Unix and added io_max_combine_limit, a postmaster-context GUC that controls shared-memory allocation at startup. The two parameters work together: io_max_combine_limit sets the ceiling at startup, while io_combine_limit can be tuned per session beneath it. A critical operational gotcha: if io_combine_limit exceeds io_max_combine_limit, the clamp is silent — SHOW reports the requested value, not the effective one. The default 128kB is well-calibrated for most hardware, but analytics workloads on high-latency network-attached storage (EBS, etc.) may benefit from larger values. Tuning should be validated via pg_stat_io.read_bytes / reads and read_time.

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