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Better observability with Dtrace, mdb, iostat, and vmstat on illumos-based systems for sure. Also simple logic dictatates that if something is made on something for that certain something, it's going to run best on that something. Linux has ZFS, but it's grafted on and the illumos POSIX layer is emulated in that sense. Further, Linux's version of OpenZFS will always lag behind fixes and features in the illumos-based systems; even FreeBSD usually contains newer vesions of OpenZFS sooner than ZFS on Linux does. Linux is just "the last hole on the porting flute".

Also, experience tells me that illumos and FreeBSD based systems will always perform faster than Linux with regads to ZFS but I'd have to publish a full line of benchmarks and I bet even those would get vehemently disputed because Linux is all the rage right now, so that's a lost cause: you have to try it for yourself and make up your own mind.