It will accumulate synchronous writes into the ZIL, and you put the ZIL on a fast SLOG vdev. But it will only do so for a limited amount of time/space, and is not meant as a proper write-back cache but rather as a means to quickly service synchronous writes.
By default asynchronous writes do not use the ZIL, and hence SLOG vdev at all. You can force it to, but that can also be a bad idea unless you have Optane drives as you're then bottlenecked by the ZIL/SLOG.
Not really.
It will accumulate synchronous writes into the ZIL, and you put the ZIL on a fast SLOG vdev. But it will only do so for a limited amount of time/space, and is not meant as a proper write-back cache but rather as a means to quickly service synchronous writes.
By default asynchronous writes do not use the ZIL, and hence SLOG vdev at all. You can force it to, but that can also be a bad idea unless you have Optane drives as you're then bottlenecked by the ZIL/SLOG.