Date: Fri, 14 Nov 2008 06:59:33 +1000 From: Danny Carroll <fbsd@dannysplace.net> To: Scott Long <scottl@samsco.org> Cc: freebsd-fs@freebsd.org, freebsd-hardware@freebsd.org Subject: Re: Areca vs. ZFS performance testing. Message-ID: <491C9535.3030504@dannysplace.net> In-Reply-To: <491C5AA7.1030004@samsco.org> References: <490A782F.9060406@dannysplace.net> <20081031033208.GA21220@icarus.home.lan> <490A849C.7030009@dannysplace.net> <20081031043412.GA22289@icarus.home.lan> <490A8FAD.8060009@dannysplace.net> <491BBF38.9010908@dannysplace.net> <491C5AA7.1030004@samsco.org>
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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
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