building a RAID system

Joel Jaeggli joelja at darkwing.uoregon.edu
Fri Oct 10 07:59:22 EDT 2003


On Fri, 10 Oct 2003 pesch at attglobal.net wrote:

> 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?

size-wise software raid (I'm talking specifically about linux here) scales
far better than most hardware raid controllers (san subsystems are another
kettle of fish entirely), among other reasons because you can spread the
disks out between multiple controllers.

> 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}...............:
> > _______________________________________________
> > Beowulf mailing list, Beowulf at beowulf.org
> > To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
> 
> _______________________________________________
> Beowulf mailing list, Beowulf at beowulf.org
> To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
> 

-- 
-------------------------------------------------------------------------- 
Joel Jaeggli  	       Unix Consulting 	       joelja at darkwing.uoregon.edu    
GPG Key Fingerprint:     5C6E 0104 BAF0 40B0 5BD3 C38B F000 35AB B67F 56B2


_______________________________________________
Beowulf mailing list, Beowulf at beowulf.org
To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf



More information about the Beowulf mailing list