building a RAID system

Jakob Oestergaard jakob at unthought.net
Fri Oct 10 03:24:15 EDT 2003


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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