Date: Sun, 30 Mar 2003 12:01:17 -0500 From: Chuck Swiger <cswiger@mac.com> To: freebsd-stable@freebsd.org Cc: Lukas Ertl <l.ertl@univie.ac.at> Subject: Re: vinum performance Message-ID: <3E8722DD.5050703@mac.com> In-Reply-To: <20030330175605.E23911@leelou.in.tern> References: <20030330125138.K23911@leelou.in.tern> <3E870CC7.5000204@mac.com> <20030330175605.E23911@leelou.in.tern>
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Lukas Ertl wrote: [ ... ] >> There are three goals or priorities to choose from when configuring >> RAID: performance, reliability, and cost. What are yours? > > I just wanted to test the performance of these drives and of vinum; I had > no goals to reach. Testing something to see what happens is a goal in and of itself. Nevertheless, my question wasn't idle: you probably will find that if you choose a goal like, "I want to set up a RAID volume that has really good performance", that you learn more from your testing. >> Also, what tasks you intend to use the RAID filesystem for are critical >> to consider, even if the answer is simply "undifferentiated >> general-purpose storage". In particular, RAID-5 write performance is >> going to be slow, even with RAID hardware support which offloads the >> parity calculations from the system CPU(s). RAID-5 is best suited for >> read-mostly or read-only volumes, where you value cost more than >> performance. > > Ok. But I still don't understand why RAID 5 write performance is _so_ bad. RAID-5 trades performance for both cost and for reliability. Actually, most forms of RAID trade performance OR cost for reliability, but RAID-5 does both and thus is generally slower than a single drive. Especially when you're got lots of multithreaded small writes; that is to say, doing an I/O benchmark, which is designed to saturate the I/O system deliberately, and tends to have an even balance of reads to writes, is pretty much a worst-case usage scenario for RAID-5. That doesn't mean that RAID-5 isn't useful, and it can perform okay under light to moderate I/O loads, but RAID-5 degrades badly under high write loads. > The CPU is not the bottle neck, it's rather bored. And I don't understand > why RAID 0 doesn't give a big boost at all. Is the ahc driver known to be > slow? FreeBSD supports Adaptec hardware very well, in general. Or is that vice versa? >> Um, that is a dual-channel card, and you're splitting drives onto both >> channels, right? > > Yes, it is dual channel, but the disks I'm testing are all connected to > the same channel. Bad layout? Highly non-optimal?, depending on how you wish to look at it. :-) -- -Chuck PS: You should also consider what happens if a drive fails; what happens to the performance then? If you've got hot-swappable hardware, try yanking a disk or two...
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