[plug] RAID on a gigabyte 7N400p2 MoBo

Craig Ringer craig at postnewspapers.com.au
Sat Dec 17 16:52:38 WST 2005

Brad Campbell wrote:
> Jonathan Young wrote:
>> I have to say that I have personally always preferred hardware when it 
>> comes to RAID, but have never used onboard.  I prefer a standalone 
>> controller with a separate BIOS onboard and that has never given me 
>> any trouble.  In my experience RAID has only been a headache when 
>> either a software solution or a cheaply produced motherboard 
>> integrated controller have been used.
> Ok, let's just get one thing straight please.. onboard RAID is *not* 
> hardware raid. It's a cheesy controller with a BIOS that *emulates* 
> raid.. just like 80% of the "hardware" RAID cards on the market these 
> days (/me waves at highpoint).

Thanks ... I've been struggling not to pipe up with this (again) in this 
discussion, and I'm relieved someone else did ;-)

The key point is that cheap onboard ATA/SATA RAID controllers are 
usually just a plain old controller with a few BIOS hooks to permit 
booting off the RAID volume. When the OS has booted to that point, the 
hardware driver for the device is loaded and operation switches over to 
software RAID implemented in the driver.

It's not necessarily a bad approach, except in that each manufacturer 
seems to insist on using a different and incompatible on-disk format for 
more complex RAID types. This means you can't just install a "generic" 
SW RAID driver to read the array off your dead machine in another box, 
which is unfortunate. Similarly, it complicates upgrades and can make 
dual-booting rather painful. The devices are also not "RAID" in any 
sense when used under any OS the manufacturer doesn't provide drivers for.

> If you must share an array with Windows and Linux on the same box, then 
> RAID-1 or RAID-0 using the onboard or an addon card (any SIL 3112 or 
> similar) will do the job, just please remember it's not hardware raid..

Actually, you can quite possibly use any RAID format the onboard device 
supports, so long as dmraid supports that controller's on-disk format. 
You can consider dmraid to be a "generic" software RAID driver with the 
capability of using the on-disk formats of quite a lot of different 
Windows software RAID driver implementations ("onboard RAID controllers").

My understanding is that these days (recent 2.6.x), Linux software RAID 
is really just another dmraid on-disk format that's compatible with the 
old `md' driver's format.

Craig Ringer

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