[Leaplist] [2/2] CentOS install -- Fake RAID (FRAID) and DeviceMapper (dmraid)

[Hank Lambert]

Bryan J. Smith wrote:
> This is most likely "Fake RAID" (FRAID).
>
> I.e., the mainboard firmware has 16-bit BIOS Extended Int13h Disk
> Services for boot-time support.  But once the 32/64-bit OS kernel
> loads, they are useless, and you need a 100% software driver to take
> over.
>
> E.g., when you installed Windows XP, you pressing F6 at install-time
> (unless the FRAID driver was included by Microsoft).
>   
Yep. Drivers on a floppy supplied from Soyo.
> Instead of /dev/hda (or /dev/sda) and /dev/hdb (or /dev/sdb), you'll
> see /dev/mapper or /dev/dm* or possibly a vendor-specific /dev/nvmap*
> or other device (actually, you may see variations of all three). 
> That's the dmraid setting up remapped devices so the kernel can use
> the organization.
>   
That's exactly what I saw. I was confused as to why I saw dev/mapper 
instead of dev/hda*
>   
>> I did not select RAID on this install,
>>     
>
> What do you mean "I did not select RAID on this install"?
> Did you mean you disabled it in the BIOS?
>
> In any case, if the disks have had a previous FRAID organization on
> them, the Red Hat Anaconda installer (RHEL 5) will detect that and
> load dmraid.  Once dmraid is loaded, it will stripe (RAID-0), mirror
> (RAID-1), stripe+mirror (RAID-10), etc... as Anaconda detected.
>   
Yes, I meant I disabled the onboard raid (fraid) in the system bios.
> Lesson:  
>
> If you use FRAID, understand what you're getting into.
>
> Under Windows XP/2003, this requires you to have the "driver disk"
> and hit "F6" at boot.  Otherwise its installer just panics with "no
> hard drive."
>
> Under newer Linux 2.6 kernels with dmraid, it will detect RAID
> organization.  You either have to have all disks _and_ the BIOS
> enabled so it can boot, or you have to disable the setting in the
> BIOS and get rid of the organization so Linux won't see it.
>
> Again, unlike Windows (which _destroys_ or at least "looses" FRAID
> volumes on the slightest case of "lack of driver" -- I've seen that
> too many times), Linux's DeviceMapper RAID (dmraid) is extremely
> intelligent and can "detect" FRAID volumes, even when they are no
> longer on the original controller with that FRAID driver's
> assumptions!
>
> It's an outstanding feature, but you have to know what you're doing. 
> If you don't want to deal with it, get a $120, 2-channel SATA 3Ware
> 9650SE-2LP (PCIe x1 true hardware RAID).
>   
Thanks for the explanation. This details exactly what happened. I wasn't 
sure what was going on, but it didn't look familiar. One thing I 
learned, I will never use Windows fraid again.