Neil or somebody? Actual ZFS developers? Taking feedback here? ;-)
While I was putting my poor little server through cruel and unusual punishment as described in my post a moment ago, I noticed something unexpected: I expected that while I'm stressing my log device by infinite sync writes, my primary storage devices would also be busy(ish). Not really busy, but not totally idle either. Since the primary storage is a stripe of spindle mirrors, obviously it can handle much more sustainable throughput than the individual log device, but the log device can respond with smaller latency. What I noticed was this: For several seconds, *only* the log device is busy. Then it stops, and for maybe 0.5 secs *only* the primary storage disks are busy. Repeat, recycle. I expected to see the log device busy nonstop. And the spindle disks blinking lightly. As long as the spindle disks are idle, why wait for a larger TXG to be built? Why not flush out smaller TXG's as long as the disks are idle? But worse yet . During the 1-second (or 0.5 second) that the spindle disks are busy, why stop the log device? (Presumably also stopping my application that's doing all the writing.) This means, if I'm doing zillions of *tiny* sync writes, I will get the best performance with the dedicated log device present. But if I'm doing large sync writes, I would actually get better performance without the log device at all. Or else . add just as many log devices as I have primary storage devices. Which seems kind of crazy.
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