[Techtalk] RAID data recovery w/ hardware controller

[Rudy Zijlstra]

Hi Carla,

On Tue, 12 Feb 2008, Carla Schroder wrote:

> Another RAID question- suppose you have a nice hardware RAID controller and it
> fails. How do you recover your data? Do you need an identical controller to
> replace it, or can the disks be read by some other means?

I'll answer this based on the AMI SCSI raid controllers i'm using myself. 
I have either the same or a later version in spare for situations like 
this. In other words, you either need a controller from the same or a 
later generation. The later controllers will read and handle raid sets 
from an earlier generation. The reverse is not true....

> I'm figuring out
> the pros and cons with Linux software RAID; with Linux RAID you can stuff
> your drives in any Linux box and re-construct the array, assuming the drives
> are not damaged and the data not corrupted. Very nice for moving to a new
> box, or recovering from some other component failure that doesn't affect the
> hard drives.

I use both HW raid and SW raid. Generally speaking i'm using HW raid on 
boxes with high availability requirements, and use them for system 
partition. big storage RAID is then SW raid. Reasons to use HW raid:
-1- easy identification of failed disk
-2- no problems to boot from a raid partition
-3- configuration / recovery can be done with no OS active

Examples: i have a IBM x-series server with 4 SCSI HDD. i tried to use SW 
raid on that, and in order to have a simple raid config, used partitioned 
raid driver. I subsequently ended up creating a specific boot CD, as no 
disk based bootloader supported this configuration... Both grub and lilo 
barfed on it :(

When i had the opportunity to get some modern SCSI-320 raid controllers 
for a sensible price i ran for it... much easier to maintain solution.

Also, i've found the beeping that ensues on a disk failure to be a very 
usefull warning sign :)

On SW raid, identifying which HDD has failed can be an issue. I've once 
lost a 1.5T array because we mis-identified which HDD had failed.... All 
data on the array was lost.

On the HW raid, when i accidentally disconnect the wrong disk, i can force 
that disk online, and incur no data loss. I've so far found no way to do 
that on SW raid... can be my problem though, as mdadm still has some 
secrets for me.

>
> Carla

Cheers,

Rudy