On Nov 13, 2008, at 21:59, Danny Carroll wrote:
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.
I have noticed that my 3ware controllers, after updating firmware
recently, have removed the JBOD option entirely, classifying it as
something you wouldn't want to do with that kind of hardware anyway. I
believed then, and even more so now, they are correct.
Use the RAID-0 disk trick to be able to utilize the controller cache.
And regarding write-back vs write-through; I believe write-through is
equvivalent to disabling controller write cache, however it WILL cache
the writes in order to respond to future reads of the data being
written. I would guess, but I don't know, that this also goes for disk-
level caches too, though, so it probably doesn't matter.
/Eirik
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