Re: md127 auto created when use "-B" to build a legacy array without superblocks

[NeilBrown <neilb@suse.com>]

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On Tue, Jan 26 2016, yizhan wrote:

> On 10/02/2015 03:55 PM, Neil Brown wrote:
>> Yi Zhang <yizhan@redhat.com> writes:
>>
>>> Hi Neil
>>>
>>> When testing 00raid1, found the md127 auto created when use "-B" to build a legacy array without superblocks, is it reasonable?
>> This happens because udev notices a new device has appeared, looks
>> inside it, sees that it could be part of an md array, and so runs
>> "mdadm --incremental" on it.
> Hi Neil
> I tried the mdadm --incremental, but it doesn't work, below is the 
> operation.
> # mdadm -D /dev/md0
> /dev/md0:
>          Version :
>    Creation Time : Tue Jan 26 03:16:17 2016
>       Raid Level : raid1
>       Array Size : 2097152 (2.00 GiB 2.15 GB)
>    Used Dev Size : 2097152 (2.00 GiB 2.15 GB)
>     Raid Devices : 2
>    Total Devices : 2
>
>            State : clean
>   Active Devices : 2
> Working Devices : 2
>   Failed Devices : 0
>    Spare Devices : 0
>
>      Number   Major   Minor   RaidDevice State
>         0       7        0        0      active sync   /dev/loop0
>         1       7        1        1      active sync   /dev/loop1
> # mdadm -D /dev/md127
> /dev/md127:
>          Version : 1.2
>       Raid Level : raid0
>    Total Devices : 1
>      Persistence : Superblock is persistent
>
>            State : inactive
>
>             Name : 0
>             UUID : 13ada118:129135ff:686de7b7:4cb71d6d
>           Events : 17
>
>      Number   Major   Minor   RaidDevice
>
>         -       9        0        -        /dev/md0
> # mdadm -I /dev/md127
> mdadm: /dev/md127 is not part of an md array.
> # mdadm -I /dev/md0
> mdadm: cannot reopen /dev/md0: Device or resource busy.
>
> After I stopped md127, the md0 can be used again.

If, after stopping md127, you try

  mdadm -I /dev/md0

again it will start /dev/md127 again.  This is what udev does which
causes md127 to appear in the first place.

NeilBrown


>
> Thanks
> Yi
>> Sometimes we want udev to do that.  Sometimes we don't.
>> There is no easy way for udev to know what we want.
>>
>> NeilBrown
>>
>>
>>> pls check below detailed info:
>>>
>>> + mdadm -CR /dev/md0 --level=raid1 -n3 /dev/loop0 /dev/loop1 /dev/loop2
>>> mdadm: /dev/loop0 appears to contain an ext2fs file system
>>>         size=58368K  mtime=Thu Jan  1 08:00:00 1970
>>> mdadm: Note: this array has metadata at the start and
>>>      may not be suitable as a boot device.  If you plan to
>>>      store '/boot' on this device please ensure that
>>>      your boot-loader understands md/v1.x metadata, or use
>>>      --metadata=0.90
>>> mdadm: /dev/loop1 appears to contain an ext2fs file system
>>>         size=38912K  mtime=Thu Jan  1 08:00:00 1970
>>> mdadm: Defaulting to version 1.2 metadata
>>> mdadm: array /dev/md0 started.
>>> + mdadm --wait /dev/md0
>>> + cat /proc/mdstat
>>> Personalities : [raid6] [raid5] [raid4] [raid1]
>>> md0 : active raid1 loop2[2] loop1[1] loop0[0]
>>>        19968 blocks super 1.2 [3/3] [UUU]
>>>        
>>> unused devices: <none>
>>> + mdadm -S /dev/md0
>>> mdadm: stopped /dev/md0
>>> + mdadm -B /dev/md0 -l raid1 -n2 /dev/loop0 /dev/loop1
>>> mdadm: array /dev/md0 built and started.
>>> + sleep 2
>>> + cat /proc/mdstat
>>> Personalities : [raid6] [raid5] [raid4] [raid1]
>>> md127 : inactive md0[0](S)
>>>        19968 blocks super 1.2
>>>         
>>> md0 : active raid1 loop1[1] loop0[0]
>>>        20000 blocks super non-persistent [2/2] [UU]
>>>        
>>> unused devices: <none>
>>> [root@dhcp-12-171 bug]# uname -r
>>> 4.2.0
>>> [root@dhcp-12-171 bug]# mdadm -D /dev/md0
>>> /dev/md0:
>>>          Version :
>>>    Creation Time : Mon Sep  7 20:21:20 2015
>>>       Raid Level : raid1
>>>       Array Size : 20000 (19.53 MiB 20.48 MB)
>>>    Used Dev Size : 20000 (19.53 MiB 20.48 MB)
>>>     Raid Devices : 2
>>>    Total Devices : 2
>>>
>>>            State : clean
>>>   Active Devices : 2
>>> Working Devices : 2
>>>   Failed Devices : 0
>>>    Spare Devices : 0
>>>
>>>      Number   Major   Minor   RaidDevice State
>>>         0       7        0        0      active sync   /dev/loop0
>>>         1       7        1        1      active sync   /dev/loop1
>>> [root@dhcp-12-171 bug]# mdadm -D /dev/md127
>>> /dev/md127:
>>>          Version : 1.2
>>>       Raid Level : raid0
>>>    Total Devices : 1
>>>      Persistence : Superblock is persistent
>>>
>>>            State : inactive
>>>
>>>             Name : dhcp-12-171.nay.redhat.com:0  (local to host dhcp-12-171.nay.redhat.com)
>>>             UUID : 40ace956:a9dd0793:f4984d2b:8431b92b
>>>           Events : 17
>>>
>>>      Number   Major   Minor   RaidDevice
>>>
>>>         -       9        0        -        /dev/md0
>>>
>>> Best Regards,
>>>    Yi Zhang

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