building a RAID system
pesch at attglobal.net
pesch at attglobal.net
Fri Oct 10 13:34:41 EDT 2003
"The problem with offloading is, that while it made great sense in the
days of 1 MHz CPUs, it really doesn't make a noticable difference in the
load on your typical N GHz processor."
Did you have a maximum data storage size in mind? - or to put it differently: at what data size do you see the
practical limit of SW RAID?
Jakob Oestergaard wrote:
> On Thu, Oct 09, 2003 at 08:50:17PM +0200, Daniel Fernandez wrote:
> > Hi again,
> Others have already answered your other questions, I'll try to take one
> that went unanswered (as far as I can see).
> > But must be noted that HW RAID offers better response time.
> In a HW RAID setup you *add* an extra layer: the dedicated CPU on the
> RAID card. Remember, this CPU also runs software - calling it
> 'hardware RAID' in itself is misleading, it could just as well be called
> 'offloaded SW RAID'.
> The problem with offloading is, that while it made great sense in the
> days of 1 MHz CPUs, it really doesn't make a noticable difference in the
> load on your typical N GHz processor.
> However, you added a layer with your offloaded-RAID. You added one extra
> CPU in the 'chain of command' - and an inferior CPU at that. That layer
> means latency even in the most expensive cards you can imagine (and
> bottleneck in cheap cards). No matter how you look at it, as long as
> the RAID code in the kernel is fairly simple and efficient (which it
> was, last I looked), then the extra layers needed to run the PCI
> commands thru the CPU and then to the actual IDE/SCSI controller *will*
> incur latency. And unless you pick a good controller, it may even be
> your bottleneck.
> Honestly I don't know how much latency is added - it's been years since
> I toyed with offload-RAID last ;)
> I don't mean to be handwaving and spreading FUD - I'm just trying to say
> that the people who advocate SW RAID here are not necessarily smoking
> crack - there are very good reasons why SW RAID will outperform HW RAID
> in many scenarios.
> > HW raid offers hotswap capability and offload our work instead of
> > maintaining a SW raid solution ...we'll see ;)
> That, is probably the best reason I know of for choosing hardware RAID.
> And depending on who you will have administering your system, it can be
> a very important difference.
> There are certainly scenarios where you will be willing to trade a lot
> of performance for a blinking LED marking the failed disk - I am not
> : jakob at unthought.net : And I see the elder races, :
> :.........................: putrid forms of man :
> : Jakob Østergaard : See him rise and claim the earth, :
> : OZ9ABN : his downfall is at hand. :
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