[CentOS] Re: IDE RAID support

[Bryan J. Smith]

Aleksandar Milivojevic <alex at milivojevic.org> wrote:
> Exactly.  The rule of the thumb is, if you setup RAID in
> BIOS, and when you boot Linux still sees individual drives,
> you don't have real RAID controller.

That's because when the BIOS is running, the CPU is in
Real86 (8086/8088) mode and can use the 16-bit RAID code.

If you boot DOS-based Windows (Windows 95/98/ME), its
386Enhanced (and yes, this is how Windows 95/98/ME still
run!) mode constantly switches between Real86 and
Protected386.  It will use the Real86 mode to access the
RAID in "compatibility" mode until you load the 100%
_software_ FRAID driver (all the 32-bit RAID code is
in the software driver, *0* on-board intelligence).

In NT-based Windows (including 2000/XP/2003), it is like
Linux and uses Protected386 mode at all times.  Thus is
requires the 100% _software_ FRAID driver.  If the NT
version has support for the ATA chipset the FRAID card
is based on, just like Linux, you will only see the
"raw" drives.  You need the FRAID driver to fool it,
because your CPU is still driving all the disk access.

"Real hardware" RAID cards don't use software, they use
an on-board intelligence like an ASIC or Microcontroller.
Those boards are totally drive by that on-board intelligence,
including a full firmware and embedded OS with the RAId logic.
Your CPU _never_ directly accesses the disk.  In fact, the
OS driver is rather _simplistic_ (simple block transfers,
possibly a few status commands) because the on-board
ASIC/microcontroller handles queuing, caching/buffering,
etc...

> That fake RAID stuff has only one purpuse.  To allow users
> of Windows XP Home/Professional to have software RAID.  If
> Microsoft shipped software RAID drivers with Home/Professional
> (like they do with Server),

??? I thought XP Pro can use LDM and striping/mirroring, just
like Windows 2000 Pro ???

> those fake RAID controllers would never exist.  Or to be more
> correct, the controllers would exist (since they are basically
> just regular SATA controllers), but you wouldn't see word RAID
> mentioned anywhere on them, and there would be no RAID settings

> in their BIOS.

It's a marketing gimmick, with or without Microsoft's support.



-- 
Bryan J. Smith                 mailto:b.j.smith at ieee.org
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