[svlug] summary of nvidia/sil soft raid & Linux soft raid

Rick Moen rick at linuxmafia.com
Mon Mar 13 13:04:07 PST 2006

[picking up the earlier but now-again-relevant SATA thread:]

Quoting Ivan Sergio Borgonovo (mail at webthatworks.it):
> Rick Moen <rick at linuxmafia.com> wrote:
> > AMCC (better known as 3Ware) is not fakeraid, but rather real
> > hardware RAID.

[Referring to 

> OK.... but if nvidia and/or sil don't have hw support for raid the
> above page would be a bit misleading.

There's still a lot of confusion about what is real hardware RAID and
what isn't.  The Nvidia and SiI chipsets are part of a growing family of
hardware capable of BIOS-assisted software RAID.  The BIOS functionality
typically supports booting from striped sets, and in most cases also the
creation of such striped sets.  Aside from that, _all_ RAID
functionality must be furnished (if at all) by driver software.  This
concept has been dubbed "fakeraid" by the denizens of
comp.os.linux.hardware, and I've adopted the term for my page because
it's a useful distinction.

Classic hardware RAID, by contrast, has a dedicated RISC coprocessor (and
RAM) that handles the RAID read/write, error-detecting/correcting, and 
restriping operations.  One useful heuristic is that motherboard-embedded
SATA RAID is almost never genuine hardware RAID -- almost always
fakeraid.  My SATA on Linux page attempts to give further guidance.

The third type of RAID is pure software RAID, typified by the Linux
kernel's "md" driver.  Both the "md" driver and typical fakeraid designs
have similar system loading characteristics:  Generally pretty good
except for severe system loading during restriping operations.

The term "fakeraid" is admittedly mildly derisive, but the concept has
some merit.  Although both it and the "md" driver offload RAID
calculation overhead onto the host CPU, modern Linux boxes' CPUs are
typically so underused that you'd never notice (except during
restriping).  As to fakeraid vs. md driver:  The md driver tends to be
faster and more reliable, and the format design is fully open rather
than proprietary (as are many but not all fakeraid schemes).  On the
other hand, one might be forced to stick with some manufacturer's
fakeraid scheme in the case of dual-boot Windows/Linux boxes where you
want to share the existing fakeraid disk set.  (To my knowledge,
nobody's implemented "md" support for MS-Windows.)

> BTW this link on your kb/Installer page seems dead:
> http://debian-amd64.alioth.debian.org/install-images/

Fixed, thanks.  Apparently, around the time of "sarge" release, the
AMD64 ISO images moved off Alioth, over to an "unofficial" tree on

I note, also:

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