Linux Hardware RAID Configuration

[Kevin Brown]

>> If it's *really* hardware raid then you'd only see 1 disk. You can 
>> only see the disk at all through the controller, after all. So the 
>> controller isn't taking over at that level. 
> 
> This is my suspicion...and what you are saying validates what it says in:
> 
> http://www.linuxhomenetworking.com/linux-adv/raid.htm#_Toc92809999
> 
>   Hardware RAID configuration is usually done via the system BIOS
>   when the server boots up, and once configured, it is absolutely
>   transparent to Linux. Unlike software RAID, hardware RAID requires
>   entire disks to be dedicated to the purpose and when combined with
>   the fact that it usually requires faster SCSI hard disks and an
>   additional controller card, it tends to be expensive.
> 
> So I'll goof around with the bios settings a bit to make sure I have 
> them right.

Most of those onboard RAID systems for SATA are not Hardware RAID and so 
the drives will still each be visible to the host OS.

>> You may have one of the SATA hardware/software combo raid systems that 
>> have been going around (I have one). If your OS doesn't support it 
>> then you can't use it as raid. I'm
>> not at all sure of Ubuntu. 
> 
> The system doesn't have an OS yet.  I've been experimenting with a few 
> distros as I'm ready to give up on Mandrake that I'm using for eons and 
> try something else.  So far it's been mostly problems.  The straight 
> Debian 3.1 R1 hangs on the install, Ubuntu 64 bit Live CD (acquired as a 
> prize at the Dev meeting the other night, Thx!) blows up on the X 
> startup.  So I was trying the Ubuntu 64 bit installed CD to see if I 
> could work out an X config.  I've long since passed my usual tolerance 
> for this sort of 'fun'....but I digress...
 >
> As for the RAID support, Ubuntu clearly has it, but I think what they 
> are talking about is Software RAID where the hardware isn't used.

RAID support depends on the device.  Ubuntu should have drivers for some 
of the true Hardware RAID devices that exist.  Most of these are SCSI 
based devices like the Mylex RAID Controller in one of my towers or the 
PERC ones that Dell seems to like for their servers.  These are true 
Hardware RAID where the OS cannot in any way shape or form directly 
access the drives as single drives.  The drivers do allow the host OS to 
see information from the controllers for things like alerting users to 
drive failure as well as handling hot swap capabilities.

Software RAID is done completely by the Host OS in most cases. 
Basically the OS is used to create a virtual drive of several drives. 
You can still see the individual drives from the host, but manipulating 
them outside of the RAID can cause bad things (e.g. mounting a drive 
participating in a RAID configuration and writing data to it).  This is 
usually a great way to do partition type RAID where you just have 
partitions on various drives linked up to act like a single RAID device.

>> I know there are drivers and stuff for it for Linux, but I don't have 
>> any details. I believe the guys at Linux Certified will ship that way 
>> as an option. Wish I had more to tell you.