The on disk cache can still cache data for reads and they actually may actually cache data for writes as well if they are using write barriers. There is no rea

The on disk cache can still cache data for reads and they actually may actually cache data for writes as well if they are using write barriers. There is no reason for the RAID controller to not let the on-disk cache and scheduling work while it is doing writeback, it only needs acknowledgement at the end before it flushes it's non-volatile cache. This could also be something that I don't know about. Maybe in the world of disks write barriers are < some disable write caching command? Can controllers issue writes large enough to make up for the lack of caching + write barriers? I have no idea, how SATA works. Again this gets into why it is hard to know what to expect from a disk IO benchmark. You have to know how the caches are operating and there are many of them and configuration can vary.