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Since its introduction in fc34b0d9de27a, the default maintenance_io_concurrency has been larger than the default effective_io_concurrency. maintenance_io_concurrency primarily controlled prefetching done on behalf of the whole system, for operations like recovery. Therefore it makes sense for it to have a value equal to or greater than effective_io_concurrency, which controls I/O concurrency for reading a relation in a bitmap heap scan. ff79b5b2ab increased effective_io_concurrency to 16, so we'll increase maintenance_io_concurrency as well. For now, though, we'll keep the defaults of effective_io_concurrency and maintenance_io_concurrency equal to one another (16). On fast, high IOPs systems, significantly higher values of maintenance_io_concurrency are observably beneficial [1]. However, such values would flood low IOPs systems and increase overall system I/O latency. It is worth mentioning that since 9256822608f and c3e775e608f, maintenance_io_concurrency also controls the I/O concurrency of each vacuum worker. Since many autovacuum workers may be simultaneously issuing I/Os, we want to keep maintenance_io_concurrency appropriately conservative. [1] https://postgr.es/m/c5d52837-6256-0556-ac8c-d6d3d558820a%40enterprisedb.com Suggested-by: Jakub Wartak <jakub.wartak@enterprisedb.com> Discussion: https://postgr.es/m/CAKZiRmxdHQaU%2B2Zpe6d%3Dx%3D0vigJ1sfWwwVYLJAf%3Dud_wQ_VcUw%40mail.gmail.com