mdstat - what is this?

[Michael Hipp]

Mike Reinehr wrote:
> On Saturday 30 September 2006 16:48, Michael Hipp wrote:
>> Mike Reinehr wrote:
>>> On Saturday 30 September 2006 08:08, Michael Hipp wrote:
>>>> What's the md255 stuff below? I didn't create any such device.
>>>>
>>>> Thanks,
>>>> Michael
>>>>
>>>> # cat /proc/mdstat
>>>> Personalities : [raid1]
>>>> md255 : active raid1 dm-4[1] dm-3[0]     <---
>>>>        136351616 blocks [2/2] [UU]        <---
>>>>
>>>> md3 : active raid1 hdc2[1]
>>>>        18787904 blocks [2/1] [_U]
>>>>
>>>> md2 : active raid1 hdc1[1]
>>>>        104320 blocks [2/1] [_U]
>>>>
>>>> md1 : active raid1 hda4[0] hdc4[1]
>>>>        136351616 blocks [2/2] [UU]
>>>>
>>>> md0 : active raid1 hda3[0] hdc3[1]
>>>>        1044160 blocks [2/2] [UU]
>>> Michael,
>>>
>>> md255 looks suspiciously like md1. Have you tried using
>>> `mdadm --query --detail /dev/md255` to learn more about it? Are there any
>>> entries in your /dev directory named dm-?? Are you using udev? Perhaps
>>> this is some peculiar behavior of udev.
>>>
>>> md now has persistent superblocks which will cause an array to be built
>>> on boot-up whether you have it defined in /etc/mdadm/mdadm.conf or not.
>>> Perhaps this is a leftover from some earlier raid array that you had set
>>> up once, but did not erase the superblock (I think that's unlikely, too,
>>> but you never know.)
>> Thanks. The md255 ghost seems to appear at random and fade quietly away
>> sometime later. It now looks like md0. This box has never had RAID on it
>> - a severe oversight I'm in process of correcting. Anyway, it doesn't
>> seem to be hurting anything so my pseudo-panic at seeing this interloper
>> in my beautiful md stable was probably unfounded.
>>
>> Thanks,
>> Michael
> 
> Michael,
> 
> If you're just now setting up RAID for the first time, I would like to make 
> two suggestions, but accept my apology if you're already there & done. First, 
> repartition & use cfdisk to set the partition type to (FD) Linux RAID 
> Autodetect. And, two, use the newer raid tool mdadm to manage your RAID 
> array. (Of course, you have to have to include md support in your kernel, 
> too.)

Thanks, already done. It was astonishingly easy on Ubuntu.

Michael