[mythtv-users] RAID versus Drobo

Michael Heironimus mkh01 at earthlink.net
Sun Sep 9 19:35:32 UTC 2007


On Sun, Sep 09, 2007 at 02:52:24PM -0400, Eric Robinson wrote:
> Andy Burns wrote:
> > On 09/09/2007 17:15, Eric Robinson wrote:
> >
> >   
> >> clarify things quite a bit.  I would most definitely be doing a software
> >> [fake] RAID-5 setup in Linux.  Some digging revealed a few links:
> >>     
> >
> > on linux, raid with mdadm isn't fake, it's software raid.
> > dmraid is what is referred to as fake raid.
> > then of course there's hardware raid.
> >   
> So what then is the difference between fake raid and software raid?

"fakeraid" in Linux refers to an entire class of controllers. They use a
proprietary software RAID system within the driver and a special BIOS
that knows how to boot from that proprietary software RAID. They are
normally advertised and sold as RAID controllers with little or no
mention of software RAID, but aside from the Windows driver and BIOS
they're ordinary IDE/SATA controllers. Most of the Linux drivers to
support these proprietary RAID volumes have been reverse-engineered by
the community.

"Software RAID" in Linux refers to the standard kernel software RAID
system, controlled through mdadm. It runs above the HBA drivers, not
inside them. It can theoretically work with just about any combination
of controllers and drives on any hardware platform, though of course
mixing and matching vastly different devices is probably not a good
idea. I don't think you can really boot a kernel off a software RAID
volume, but there are plenty of documents describing how to create
redundancy for /boot and the boot loader so that's not a significant
problem.

You'll find similar terminology in the BSDs, except that they've shown
even less interest in supporting fakeraid cards as anything more than
ordinary IDE controllers.

-- 
Michael Heironimus


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