Re: [gentoo-user] Problems with Nvidia fake raid array

[Jeff Cranmer]

On Mon, 2011-07-18 at 22:29 -0400, Michael Orlitzky wrote:
>
> Make sure your kernel supports RAID, and RAID5 (they're separate
> options). Then emerge mdadm. Once you get it up and running once, you
> can dump the current config to /etc/mdadm.conf so you don't have to
> assemble it again. Then add mdadm to the boot runlevel.
>
I'm Ok so far - Raid and Raid5 options are both already compiled into
the kernel, and mdadm is in the boot runlevel.

> # mdadm --assemble --help
> Usage: mdadm --assemble device options...
>        mdadm --assemble --scan options...
>
> This usage assembles one or more raid arrays from pre-existing
> components. For each array, mdadm needs to know the md device, the
> identity of the array, and a number of sub devices. These can be found
> in a number of ways.

> The md device is either given on the command line or is found listed
> in the config file. The array identity is determined either from the
> --uuid or --super-minor commandline arguments, from the config file,
> or from the first component device on the command line.
>
> The different combinations of these are as follows:
> If the --scan option is not given, then only devices and identities
> listed on the command line are considered.
>
> The first device will be the array device, and the remainder will be
> examined when looking for components.
>
> If an explicit identity is given with --uuid or --super-minor, then
> only devices with a superblock which matches that identity is
> considered, otherwise every device listed is considered.
>
> If the --scan option is given, and no devices are listed, then
> every array listed in the config file is considered for assembly.
> The identity of candidate devices are determined from the config file.
>
> If the --scan option is given as well as one or more devices, then
> Those devices are md devices that are to be assembled. Their identity
> and components are determined from the config file.
>
> If mdadm can not find all of the components for an array, it will
> assemble it but not activate it unless --run or --scan is given. To
> preserve this behaviour even with --scan, add --no-degraded. Note that
> "all of the components" means as many as were present the last time the
> array was running as recorded in the superblock. If the array was
> already degraded, and the missing device is not a new problem, it will
> still be assembled. It is only newly missing devices that cause the
> array not to be started.

Pardon my additional questions before taking the plunge here.  

So, given that I have three devices, /dev/sda, /dev/sdb and /dev/sdc, if
I run the command mdadm --assemble --scan, would this find all the
components and create a /dev/md0 disk without damaging the contents of
the original RAID array?

The only item in /dev/mapper is th default 'control' entry.  There is
a /dev/md0 item already listed, but presently when I try to mount it, it
reports that it is unable to read the superblock.  Would the command
above fix this?

Where is the config file mentioned in your e-mail, and do I need to edit
it first to add the three raid disks?

Thanks

Jeff