Marion, this is a common misintrepetation :

        "Anyway, I've also read that if ZFS notices it's using "slices" instead 
of
        whole disks, it will not enable/use the write cache. "

The reality is that

        ZFS turns on the write cache when it owns the
        whole disk.

_Independantly_ of that,
        
        ZFS flushes the write cache when ZFS needs to insure 
        that data reaches stable storage.

The point is that the flushes occur whether or not ZFS turned
the caches on or not (caches might be turned on by some
other means outside the visibility of ZFS).

The problem is that the flush cache command means 2
different things to the 2 components.

To ZFS :

        "put on stable storage"

To Storage:

        "flush the cache"

Until we get this  house in order,  storage needs  to ignore
the requests.

-r

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