building a RAID system

pesch at attglobal.net pesch at attglobal.net
Fri Oct 10 13:34:41 EDT 2003


You write:

"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?

Paul

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
> kidding.
>
> Cheers,
>
> --
> ................................................................
> :   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.           :
> :.........................:............{Konkhra}...............:
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