[CentOS] Installing CentOS on RAID PATA not SATA - A8V

Thu May 19 11:33:53 UTC 2005
Bryan J. Smith <b.j.smith at ieee.org>

On Wed, 2005-05-18 at 20:23 -0700, Bogdan Nicolescu wrote:
> My first attempt at RAID...

I purposely call it Fake/Free RAID (FRAID) for a reason.
It is a _regular_ ATA controller with *0* extra hardware.
There is a "trick" 16-bit BIOS that can setup some RAID
organization.

It is _useless_ once a 32/64-bit OS boots.

> Tried a different distribution with the latest kernel,
> and indeed the PTA drives on the Promise controller
> were recognized but they were recognized as two
> separate drives while the controller was in RAID mode
> (rather than IDE mode).

Correct.  Because the system sees the "raw" ATA channels.
There must be a "trick" 32/64-bit driver to match the
"trick" 16-bit BIOS.

This "trick" 32/64-bit driver contains _all_ of the
RAID functionality.  *0* is in the hardware, because the
hardware has *0* RAID firmware other than the ultra-simple
stripe/mirror access when the 16-bit BIOS is booting.

About the same 2-3 companies write these drivers, and
_all_ vendors license that code.  Because it is the core
IP of those companies, they will _never_ release it.
As such, the drivers are _always_ "closed source" and
really only available for Windows.  The few companies that 
do release Linux drivers release kernel version-specific
ones that are binary-only, no source code.

There is a "clean-room" GPL ATA RAID driver in ataraid.c.
And then interface drivers in hptraid.c, pdcraid.c and
silraid.c, but they _rarely_ work with most "FRAID"
cards accept for very old ones.  And they are _slower_
than the OS' RAID (NT LDM, Linux LVM/MD, etc...).

> Are you saying that even if the drives are recognized,
> I will not be able to implement RAID in hardware mode?

There is _no_ "hardware" mode.  The system _always_ sees
the drives.  There must be a "trick" driver to "hide" the
drives and make them seem organized.

A _true_ hardware RAID controller _always_ has on-board
intelligence that directly controls the drives, and has
the RAID firmware on-board.  In those cases, the system
talks to that on-board intelligence (e.g., microcontroller
or ASIC) and _never_ actually sees the drives.  Because
all of the RAID intelligence is in the firmware, the OS
driver is just a basic block driver, so they are GPL.

Examples of true hardware ATA RAID include Promise SuperTrak
(_not_ the FastTrak), Adaptec 2400A/2800A, some (but not
all) LSI Logic MegaRAID and 3Ware Escalade products.

These cards cost between $125-$500+.  You will _not_
find a _true_ hardware RAID controller on a mainboard.  ;->

> Any other options installing CenOS 4 on unsupported hd
> controller besides:
> a. install CentOS 4 while drive in recognized hd
> controller
> b. get new kernel 
> c. compile/install new kernel (without pulling my hair
> out)
> d. move drives to unsupported hd controller
> e. set controller to IDE mode
> f. boot using new kernel (while praying)
> g. do software RAID

Yes, software RAID is typically _faster_ than these FRAID cards.
Why?  Because instead of using the logic of the FRAID controller,
you get the logic of the OS' buffering.  The OS' buffering will
commit writes and interleave reads far better than a FRAID
controller's "trick" driver logic.


-- 
Bryan J. Smith                                 b.j.smith at ieee.org 
----------------------------------------------------------------- 
Beware of those who define their preference in terms of hate of
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