Areca vs. ZFS performance testing.
fbsd at dannysplace.net
Thu Nov 13 12:59:43 PST 2008
Scott Long wrote:
> The Areca controller likely doesn't buffer/cache for disks in JBOD mode,
> as others in this thread have stated. Without buffering, simple disk
> controllers will almost always be faster than accelerated raid
> controllers because the accelerated controllers add more latency between
> the host and the disk. A simple controller will directly funnel data
> from the host to the disk as soon as it receives a command. An
> accelerated controller, however, has a CPU and a mini-OS on it that has
> to schedule the work coming from the host and handle its own tasks and
> interrupts. This adds latency that quickly adds up under benchmarks.
> Your numbers clearly demonstrate this.
That's nice to know. I'm not sure it tells us why the Non-Cached writes
were about 8% faster though. The other thing about the "NoWriteCache"
test I performed that I neglected to mention yesterday is that I
actually panic'd the box (running out of memory). This was the first
time I have had that happen with ZFS even though in previous testing
(with cache enabled) I punished the box for a lot longer.
Perhaps the ZFS caching took over where the disk caching left off?
Could that explain why I did not see a negative difference in the
numbers between Cache enabled and Cache disabled?
One of the questions I wanted to answer for myself was just this: "Does
a battery-backed cache on an Areca card protect me when I am in JBOD
mode." If the Areca does not buffer/cache in JBOD mode then that means
the answer is no.
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