Hardware RAID disk signatures

[Bozz]

As per title, how do hardware controllers know which disk is which? I suspect they write a signature somewhere on the disk that they dont make available to the OS or user, is there any more in-depth info about it or if its done another way, how its done?

Cheers

 

[corkyg]

They don't have to. Every HDD carries its own ID.

 

[Bozz]

How does a brand new controller know an array exists that was created from two HDDs on a different controller altogether?

 

[Matthias99]

Originally posted by: Bozz
How does a brand new controller know an array exists that was created from two HDDs on a different controller altogether?

In general -- it doesn't, unless the array is from the same kind of controller. Each controller manufacturer usually has some way of tracking it, probably by writing some extra data to the boot sector or a hidden partition (if you look at the total available size of a RAID array versus the raw space on the drives added together, usually you'll see it's a tiny bit smaller.)

Some controllers (especially older ones) don't necessarily store much data, and may break if you do things like swapping drives around on the controller ports. Sometimes if you have to replace the controller, you need to specifically tell it what kind of array is supposed to be there, and only then it can correctly access it.

If you created an 'empty' array and then used software to look at the drives in a totally raw format (reading individual blocks rather than going through the filesystem), you could see what the controller wrote.

It would be really nice if there was an industry-wide standard for this. But there isn't.

 

[Lord Evermore]

In particular, don't expect the soft-RAID "hybrid" controllers used on most add-in cards and integrated into chipsets to be capable of storing anything within the chipset or CMOS or anything. It's all just written to the hard drives, which is why the RAID BIOS has to scan the drives to find any RAID arrays on every boot, instead of just checking whether it's the same drives that were there before and match the stored configuration.

 

[Bozz]

Good stuff

Thanks for the replies

 

[postmortemIA]

Originally posted by: Lord Evermore
In particular, don't expect the soft-RAID "hybrid" controllers used on most add-in cards and integrated into chipsets to be capable of storing anything within the chipset or CMOS or anything. It's all just written to the hard drives, which is why the RAID BIOS has to scan the drives to find any RAID arrays on every boot, instead of just checking whether it's the same drives that were there before and match the stored configuration.

Yep, and that is how dmraid works... dmraid is tool for linux OS that recognizes soft-RAID arrays... so it recognized my single drive as part of NVRAID array, even when array was removed and drive moved to intel- based chipset.

 

[Lord Evermore]

So dmraid is basically a software version of the nvraid controller (and presumably others)? That's pretty cool. Although not really hard, since the controller is just an IDE controller with firmware to communicate with the RAID drivers.