Re: Adding disks with raid to existing raid system.

[Simon Matthews <simon.d.matthews@gmail.com>]

On Mon, Mar 15, 2010 at 9:59 PM, Michael Evans <mjevans1983@gmail.com> wrote:
> On Mon, Mar 15, 2010 at 7:53 PM, Simon Matthews
> <simon.d.matthews@gmail.com> wrote:
>> On Mon, Mar 15, 2010 at 5:36 PM, Michael Evans <mjevans1983@gmail.com> wrote:
>>> On Sun, Mar 14, 2010 at 10:23 PM, Simon Matthews
>>> <simon.d.matthews@gmail.com> wrote:
>>>> I have just built a system and have it booting off a software raid
>>>> partition. The raid sets use devices /dev/md0, /dev/hd1, /dev/md2,
>>>> /dev/md3.
>>>>
>>>> I now need to transfer some additional disks to this system. These
>>>> disks are presently in another system where they host a number of raid
>>>> sets, currently also /dev/md0 - /dev/md4.
>>>>
>>>> I need to ensure that the data on the raid set that I am adding to the
>>>> system is not lost. However, clearly, I can't have the raid sets on
>>>> these disks come up as /dev/md0-md4. How do I ensure this and have
>>>> these raid sets come up on /dev/md5 and higher?
>>>>
>>>> Simon
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>>>
>>> Either use an mdadm.conf to specify the mapping of UUID to md device
>>> (which will over-ride any auto-detected requests), or use the
>>> home-host fallback.  Obviously the administrator specifying how they'd
>>> prefer mdadm to assemble the drives is preferable.
>>
>> I'm not aware of the "home-host fallback" can you give me some pointers on this?
>>>
>>> You will probably want to regenerate your initrd; if you are using
>>> auto-assembly on root without an initrd, I highly suggest upgrading to
>>> use an initrd/initramfs.  You might find this one easy to customize
>>> for your needs if your distribution lacks one or you dislike the one
>>> it generates: http://sourceforge.net/projects/aeuio/
>>
>> Fortunately Gentoo includes mkinitrd so I can try this if other
>> methods don't work reliably.
>>
>> Simon
>>
>>>
>>
>
> man mdadm
> /host
>
> --homehost=
>
> This will override any HOMEHOST setting in the config file and
> provides the identity of the host which should be considered the home
> for any arrays.
>

I tried creating an initrd, without any great success so far (would
not boot). But while I work on that, I had another thought.

If I make the partition types for the raid components type 83 (ext3),
the kernel should not (I think) recognize and start these arrays, but
they will be stated later in the boot process by the userspace  tools
as long as there are appropriate entries in /etc/mdadm.conf.

Does this sound like a workable solution?

How can I tell what HOMEHOST parameter is on the raid sets? I don't see it with
mdadm --examine /dev/sdXX
or
mdadm --detail /dev/mdX

Simon
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