[CentOS] Re: Problem selecting installation drive : 5.2

Johnny Hughes johnny at centos.org
Tue Oct 28 13:41:22 UTC 2008

Alex H. Vandenham wrote:
> After much frustration and wasted time:
> Solution:  Additional Boot option (undocumented) : nodmraid

That either is or is not the solution ... depending on what you want :D

dmraid (Device Manager RAID) is a package that provides the ability to
recognize SOME "Fake RAID" controllers as hardware RAID.

What is FRAID:

One of the controllers that is being recognized by dmraid is the nvidia
nforce4 raid controller.

If you do "nodmraid" as a kernel switch, then you are disabling the
dmraid, so CentOS will not see the drives as RAID ... so then you can
use software RAID if you want.

If you can not see the drives at all without the nodmraid, then it means
that for some reason dmraid does not properly see the devices (or that
you did not properly set up the RAID arrays in your BIOS).

If the CentOS kernel and DMRAID does not see the devices properly as
fake RAID (dmraid) devices, then the only real option you have is to use
 them as software raid.

> On Friday 24 October 2008 06:41:07 pm Scott Silva wrote:
>> on 10-24-2008 7:25 AM Alex H. Vandenham spake the following:
>>> When doing the 5.2 installation the ONLY drive option provided is
>>> mapper/nvidia_cbjcdhfe (250G)
>>> My system has 2 SATA - 250G drives and I  want to use them in a RAID/LVM
>>> configuration.
>>> My question:
>>> How do I get the installer to let me use DiskDruid to create RAID1 arrays
>>> and then use LVM for the mount points?
>>> What I've done so far:
>>> I've verified that both drives are recognized by going to the shell
>>> screen and using fdisk to access both /dev/sda and /dev/sdb - so I know
>>> they are both available.
>>> I've also tried using the Centos 4.7 installer where I can do exactly
>>> what I want.
>>> I've accepted the default and after a full install of 5.2 there is no
>>> raid and total space is 250G.

In this case, if you have setup the devices in the BIOS as RAID-0 that
is exactly what you would expect to see if dmraid was working.  The OS
only sees 1 drive at 250GB and you are controlling the setup of the
device in the system BIOS.  Whether or not that is what you want to do,
you will need to decide.

> the default install DOES use raid but not with Software raid so /proc/mdstat 
> shows no devices.  How nvidia drivers/Linux kernel modules handle the raid 
> (and raid failures for example) remains unknown - info Anyone?
>>> I've looked at the various deployment guides and installation guides but
>>> can't find how to do this . . . .
> Chapter 8 does not mention the nodmraid option.  When that option is used, the 
> two SATA disks are listed as expected.
>>> puzzled
>>> Alex
>>> ===
>> The default install doesn't do a software raid, and if you are trying to
>> use an Nvidia onboard raid controller as a raid device in linux, you are
>> probably out of luck.
> Apparently the nvidia drivers and/or the kernel LVM modules are creating a 
> raid array - hard to know for sure without some documentation to help me 
> confirm that . . . 

Sure, if it can detect that you have the BIOS set so that the devices
are RAIDed it will detect them as one RAIDed drive.  If that is not what
you want, turn off RAID in the BIOS ... if it is what you want, great :D

>> To do software raid  with LVM over it, you have to do it all manually.
>> First creating the raid devices, and then adding the resulting md devices
>> as LVM partitions.
> Even when I created working Software RAID1 arrays with mdadm (with Centos 4.7 
> rescue), on reboot the SATA drives were used by the nvidia drivers (not 
> sure ?) to create the array with LVM VG/LV, and then md could not start the 
> software raid1 arrays.  

Again, that is what you would expect if you did not do the nodmraid
option (or turn off RAID in your BIOS).  CentOS-4 does not see that as
hardware RAID ... CentOS-5 does.

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