Comment by inigyou

They were, and still are - multiple times a year. But THE german holocaust was the most effective by orders of magnitude. And they have technology to thank for that. Computing machines (primitive as they were) made it possible to track the status of each Jew. Railways made it possible to move them. Chemical plants produced the gas in the needed quantities. Industrial incinerators made it possible to dispose of the remains. They didn't just go down to Home Depot and buy a bottle of poison gas. They had a system end-to-end as efficient as any factory.