[CentOS] AIC7901 RAID320 with CENTOS4 not recognized

Aleksandar Milivojevic alex at milivojevic.org
Sun Oct 30 17:51:21 UTC 2005

Erick Perez wrote:
> But having a "fake" bios doing raid1 isnt' more stable/better than to
> emulate raid1 in software?

Actaually no.  Linux software RAID is way more tested and stable then 
any propriatary fake RAID driver.  Since fake RAID drivers are really 
just implementation of software RAID, you do not gain absolutetly 
anything by using them.

Simply set the BIOS to JBOD mode (do not define any RAID devices in it), 
and do standard Linux software RAID thing.  Usually, it will be faster 
than using propriatary device driver.

There are only two reasons for using those fake RAIDs.

First is if you are building Windows 2000/XP home, workstation or 
professional box.  Microsoft ships software reaid drivers only for 
server version of Windows.  So those fake RAID drivers are about the 
only way to get software RAID in home, workstation and professional 
version of Windows.  Actually, if you check difference in price between 
normal (non-RAID) controller and its equivalent fake-RAID version, 
usually it is less than difference in price between Windows Pro and 
Windows Server license (otherwise, there would be no economical sense 
for buying the fake-RAID card).

Second is if you are installing more than one OS on the box (for example 
Windows and Linux) and you want both operating systems to share some or 
all of the RAID devices.

> However the main question remains, Why the adaptec 79xx driver does
> not "see" that I created a raid1 in the bios? why keeps showing me two
> "drives" instead of one?
> Cause that will mean that the linux 79xx is not "fully" compatible
> (ok, it is 99.999999% compatible) with the  AIC-7901 Adaptec 320 with
> HostRAID (fake BIOS or not).

None of the fake RAID device drivers will ever become part of mainstream 
Linux kernel.  The approach Linux takes to support them is to use device 
mapper.  Basically, during boot, Linux would read fake RAID info from 
BIOS, and than configure device mapper appropriately.  This is a generic 
approach that will work with all fake RAID controllers, using stable and 
well tested device drivers that are already part of the kernel.  If you 
are interested in doing things this way (configure RAID in BIOS and than 
use device mapper), type dmraid in Google.  Dmraid already has support 
for many fake RAID cards.

This should answer your question why 79xx device driver (the one which 
is part of standard kernel) does not see those fake RAID devices you 
created.  It simply sees tha card for what it really is.  Standard SCSI 
controller.  What you bought from Adaptec is standard SCSI controller 
and software RAID driver for Windows.  Linux comes standard with it 
(unlike non-server versions of Windows), so you don't need that 
additional software under it.  If you really want to use fake-RAID 
devices in BIOS, use dmraid to configure device mapper during boot.

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