I don't use ZFS or QNAP but Linux has its own native dm-cache and bcache implementations for creating hybrid volumes out of SSD plus hard drives. I would suspe

I don't use ZFS or QNAP but Linux has its own native dm-cache and bcache implementations for creating hybrid volumes out of SSD plus hard drives. I would suspect that QNAP is using one of them. https://en.wikipedia.org/wiki/Dm-cache https://en.wikipedia.org/wiki/Dm-cache https://en.wikipedia.org/wiki/Bcache https://en.wikipedia.org/wiki/Bcache