We've discussed previously the negative impact of large bulk operations, especially wrt WAL writes. Patch here allows maintenance operations to have their WAL generation slowed down as a replication lag prevention feature.
I believe there was originally intended to be some work on I/O rate limiting, but that hasn't happened and is in some ways orthogonal to this patch and we will likely eventually want both.
Single new parameter works very similarly to vacuum_cost_delay
wal_rate_limit_delay = Xms
slows down CLUSTER, VACUUM FULL, ALTER TABLE (rewrite & set tablespace), CREATE INDEX so basically same things we optimise WAL for and the same places where we honour maintenance_work_mem (discuss: should we add COPY, CTAS etc also?) (discuss: do we need another parameter to specify "cost"? Currently patch uses "sleep every 64kB of WAL")
VACUUM is not included, since we already have controls for that - honouring two controls would be complex and weird.
I wonder if should replace vacuum_cost/delay with say maintenance_cost/delay and use it in all maintenance activities including what you just listed out above. While the exact semantics of vacuum and other maintenance activities may differ, ISTM the final goal is just the same i.e. reduce load on the master.
