Areca vs. ZFS performance testing.

[Danny Carroll]

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.

-D