[nycbug-talk] Quasi-RAID [Was: AMD Raid question. . .]

[alex at pilosoft.com]

On Mon, 24 Oct 2005, Charles Sprickman wrote:

> On Mon, 24 Oct 2005 alex at pilosoft.com wrote:
> 
> > Hint: Most of the "hardware" raid is *not* hardware raid.
> >
> > http://linux.yyz.us/sata/faq-sata-raid.html
> >
> > Controller fired.
> 
> I'd like to fork this debate off so George can go about his business while 
> I ask tangential questions...
> 
> I've never understood the "quasi" hardware RAID that's popping up on more 
> and more mainboards.  It does not fit either of the traditional 
> definitions:
> 
> -Hardware RAID - you have either an add-on card, a ZCR card (Zero 
> Channel RAID - leverages built-in scsi controllers), or an outboard 
> scsi-scsi/ide-ide box.  All of these have a processor and drive 
> controllers onboard to handle the RAID calculations and logic.  They hide 
> the raw drives from the OS and present the configured RAID array instead.
> 
> -Software RAID - You use standard IDE/SCSI controllers and the OS deals
> with creating/maintaining the array.  The host CPU is used for all the
> XOR calculations when running RAID 5.  Booting may or may not be simple
> when a drive fails as you have to rely on the BIOS to boot off of the
> "good" drive if an element of the array fails.  This seems less complex
> with non-i386 processors as they don't have a clunky, hacked BIOS to
> deal with.
> 
> -Quasi-RAID - What George apparently has here and many people now call
> "Hardware RAID".  How does this work?  Is it basically a standard IDE
> controller?  Do they just load up a special BIOS that has enough smarts
> to know what to boot off of when there's a failure?  Why is the array
> "configured" in the BIOS?  How are the drives presented to the OS?
fakeraid is standard IDE with:
* BIOS support to boot off raid0 and broken raid1
* OS driver support to do software-raid as-configured-in-bios
* Driver presents to OS single device, and internally does multiple writes 
to reflect the chosen "raid" level

> I'd love to know more about this, as it seems like it may be appropriate
> (and cheap) in some situations...
It is almost always *better* to use software raid instead of fakeraid. 
Different vendors have different formats for storage of raid information - 
so moving the drives from one controller to another might well break your 
array. Vendor drivers are binary-only (with all problems resulting from 
that) - there are open-source reverse-engineered "drivers" for linux, that 
essentially are wrappers configuring standard softraid based on the 
BIOS-raid data on-disk. 

-alex