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Submitting IO in larger batches can be more efficient than doing so one-by-one, particularly for many small reads. It does, however, require the ReadStreamBlockNumberCB callback to abide by the restrictions of AIO batching (c.f. pgaio_enter_batchmode()). Basically, the callback may not: a) block without first calling pgaio_submit_staged(), unless a to-be-waited-on lock cannot be part of a deadlock, e.g. because it is never held while waiting for IO. b) directly or indirectly start another batch pgaio_enter_batchmode() As this requires care and is nontrivial in some cases, batching is only used with explicit opt-in. This patch adds an explicit flag (READ_STREAM_USE_BATCHING) to read_stream and uses it where appropriate. There are two cases where batching would likely be beneficial, but where we aren't using it yet: 1) bitmap heap scans, because the callback reads the VM This should soon be solved, because we are planning to remove the use of the VM, due to that not being sound. 2) The first phase of heap vacuum This could be made to support batchmode, but would require some care. Reviewed-by: Noah Misch <noah@leadboat.com> Reviewed-by: Thomas Munro <thomas.munro@gmail.com> Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt