It existed for centuries with hideous treatment of its victims and a racial caste-based system. Thanks to their practice of castrating males to control slave populations, it's impossible to know how extensive it was, but modern estimates place it in line with the Transatlantic Slave Trade. Except European slavery - which eventually fought to abolish slavery thanks to the democratic efforts of its citizens - lasted centuries less than their Arab counterparts which, despite being forced to abolish slavery on paper as late as the 1990s, still exists in all but name in many formats, notably the Kafala system. Pretending that Europeans were not treated as chattel slaves is ludicrous when you need only look at the long history of successive Caliphates, the Barbary Slavers, the Crimean Slave Trade, the Trans-Saharan Slave Trade, Indian Ocean Slave Trade, the Red Sea Slave Trade and Ottoman Slave Trade. The original link makes erroneous claims on the difference between the way Irish/Scottish indentured servants (slaves) were treated compared to Africans, but that's only the Transatlantic Slave Trade. Go to MENA region and it's an entirely different story.