[X-Unix] Hardware RAID Setup

[Stroller]

On 13 May 2010, at 14:54, Martin McCormick wrote:

> Alexandre Gauthier writes:
>> I am still curious as to if these manipulations create a software  
>> raid
>> array through OS X, or if that somehow hooks in the hardware and  
>> does it
>> there...
>
> 	It is done in hardware. Now you see 1 single drive
> instead of 2 drives. The RAID card has [features]

Yes, but how do you know it's using those features?

I'm unfamiliar with Mac RAID, but how do you know the RAID controller  
isn't offering the drives as single units, and that you're not using  
OS X's software RAID drivers to create the array?

On a BIOS-based PC this is fairly obvious. When one boots, after the  
normal BIOS messages, there is another one (white writing on the black  
background) inviting one to enter a RAID management utility. In the  
BIOS-like RAID management utility one can see the individual disks and  
create an array from them. When booted to Linux or Windows only a  
single drive is seen by the operating system, and if one wishes to add  
more drives in a new array (or reconfigure the array on which the host  
o/s resides) one must install and use the manufacturer's utility (e.g.  
tw_cli for a 3ware card) in order to do so.

Under Linux one uses a different command `mdadm` to create software  
RAID arrays. That's the Linux software RAID equivalent to 3ware's  
Linux `tw_cli` [1] or LSI's Linux `megaraid` [2], the latter of which  
operate only on hardware RAID cards. If you were using a hardware RAID  
card under Linux and you created an array using `mdadm` [3] then the o/ 
s would create a *software* RAID array out of whatever drives or  
arrays the hardware RAID card was showing it.

I am somewhat confused because you *seem* to be using the same command  
to configure your hardware array as one would use to create a software  
RAID array out of drives connected to a dumb SATA controller. This is  
*probably* Mac OS X being terribly clever and sweet and elegant and  
using the same front-end interface - Disk Utility &/or raidutil - to  
manage both kinds of driver (for both hardware and software RAID), but  
with my background I find it confusing. I would find this the  
antithesis of reassuring until I had experimented with it thoroughly  
to assure myself that hardware RAID was indeed being used, and that I  
hadn't accidentally configured software RAID on my hardware RAID card.

I hope my explanation makes sense, and I'm sorry if it's unhelpful of  
me to raise my confusion.

I could probably clear up my confusion some with some comprehensive  
Googling on the subject of Apple's hardware RAID cards, but since  
we're having this conversation, I might as well throw my confusion on  
the table for discussion.

As an exercise, if you had the time, it might be interesting for you  
to physically remove the hardware RAID card, connect the drives  
directly to the Mac's built-in regular SATA controller, and to  
configure a software RAID array. How does it behave differently?

Stroller.



[1] http://www.cyberciti.biz/files/tw_cli.8.html
[2] http://www.manpagez.com/man/8/megaraid/
[3] http://linux.die.net/man/8/mdadm