Re: Benchmarks: Linux Kernel RAID vs a Hardware RAID setup

[Brad Campbell <brad@wasp.net.au>]

Keld Jørn Simonsen wrote:

>>> It is possible that I am thinking of what you call fakeraid.
>>>  
>> I like "firmware raid" better, but the bottom line here is that the raid 
>> is still done in the system CPU. These often have little or no hardware 
>> support, such as cache so that multiple drives can be written without 
>> passing the data through the system bus more than once (and chancing 
>> change while that happens).
> 
> OK, if the definition on these controllers is that they must have support
> from the OS, then I think most of the mobos today come with hardware
> raid, that is there needs to be no support in Linux for this. Everything
> is set up via the bios, and from the linux kernel it looks like an
> ordinary disk.

The problem here is it looks like an ordinary disk to the BIOS only. Grub can use this to load the 
kernel and initrd from, but when the kernel boots it sees the controller as the simple controller it 
is with some disks connected to it. It requires something like dmraid to make the "array" visible as 
an actual array.

None of these controllers perform any of the RAID offload themselves, it's all handled by the 
driver. Windows loads using the BIOS and then hands over to the driver which handles the raid, just 
the same as linux does with dmraid.

I believe some of the ITE chipsets can handle raid-1 and possibly raid-0 internally, and the kernel 
has the ability to drive those as a raid controller, but they are somewhat rarer than the other 
fakeraid implementations out there.

Brad
-- 
Dolphins are so intelligent that within a few weeks they can
train Americans to stand at the edge of the pool and throw them
fish.

--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html