Question about preferred minor.

Question about preferred minor.

[Andrew Dunn]

I have never noticed this, my apologies for not being able to resolve
this independently.

I have this array that I am using for the primary bulk of my data and
today was the first day that I noticed this designator in the detailed
output:

storrgie@MINERVA:~$ sudo mdadm -D /dev/md2
/dev/md2:
        Version : 00.90
  Creation Time : Fri Apr 30 21:13:28 2010
     Raid Level : raid6
     Array Size : 11721071616 (11178.09 GiB 12002.38 GB)
  Used Dev Size : 1953511936 (1863.01 GiB 2000.40 GB)
   Raid Devices : 8
  Total Devices : 8
Preferred Minor : 2
    Persistence : Superblock is persistent

    Update Time : Wed Jun  2 20:42:04 2010
          State : clean
 Active Devices : 8
Working Devices : 8
 Failed Devices : 0
  Spare Devices : 0

     Chunk Size : 512K

           UUID : f340fbe8:51967d4b:6d37f269:4e81e338 (local to host MINERVA)
         Events : 0.26

    Number   Major   Minor   RaidDevice State
       0       8       65        0      active sync   /dev/sde1
       1       8       81        1      active sync   /dev/sdf1
       2       8       97        2      active sync   /dev/sdg1
       3       8      113        3      active sync   /dev/sdh1
       4       8      129        4      active sync   /dev/sdi1
       5       8      145        5      active sync   /dev/sdj1
       6       8      161        6      active sync   /dev/sdk1
       7       8      177        7      active sync   /dev/sdl1


What is the meaning of Preferred Minor : 2 ?

Thank you all.

Re: Question about preferred minor.

[Neil Brown]

On Wed, 2 Jun 2010 20:53:28 -0400
Andrew Dunn <andrew.g.dunn.dod@gmail.com> wrote:

> I have never noticed this, my apologies for not being able to resolve
> this independently.
> 
> I have this array that I am using for the primary bulk of my data and
> today was the first day that I noticed this designator in the detailed
> output:
> 
> storrgie@MINERVA:~$ sudo mdadm -D /dev/md2
> /dev/md2:
>         Version : 00.90
>   Creation Time : Fri Apr 30 21:13:28 2010
>      Raid Level : raid6
>      Array Size : 11721071616 (11178.09 GiB 12002.38 GB)
>   Used Dev Size : 1953511936 (1863.01 GiB 2000.40 GB)
>    Raid Devices : 8
>   Total Devices : 8
> Preferred Minor : 2
>     Persistence : Superblock is persistent
snip
> 
> What is the meaning of Preferred Minor : 2 ?

It means that if the array is auto-assembled, whether by the kernel with
auto-detect or by mdadm with --incremental or '-As', it will be assembled
as /dev/md2 - i.e. the device file will have a major number of 9 (meaning
an md raid array) and the minor number will be 2.

NeilBrown


> 
> Thank you all.
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Re: Question about preferred minor.

[Christian Gatzemeier]

Andrew Dunn <andrew.g.dunn.dod <at> gmail.com> writes:

> What is the meaning of Preferred Minor : 2 ?

Most of the time that array will get assembled as md2, but conflicts,
other numbers and errors stemming from differently assembled arrays can
occur.

From https://wiki.ubuntu.com/ReliableRaid:
"Mdadm is setting up arrays according to unreliable superblock
information. (Device "minor" numbers, labels and hostnames in
superblocks are not sure to be unique and can be outdated.) This is
combined with the idea of fixing the unreliability by limiting array
assembly with information from mdadm.conf. (Defining PARTITIONS, ARRAY,
HOMEHOST lines.) Consequently this forces setup tools, admins and
installers to create mdadm.conf files and subjects them to the exact
same reliability problems."

I guess this conflict prone behavior is mostly legacy behavior from the
past, or is there still an apparent reason to expose dynamic kernel
internals like the minor to userspace instead of basing device node name on
"md-UUID"?

Regards,
Christian

Re: Question about preferred minor.

[Andrew Dunn]

In the past without knowing exactly why I have experienced failed
assemblies upon a reboot. I have seen correction in this behavior by
properly populating my /etc/mdadm/mdadm.conf file.

I typically log in as root and do this:

mdadm --examine --scan --config=mdadm.conf >> /etc/mdadm/mdadm.conf

Clean up is typically needed.

So could this be an indication that my super-blocks on this particular
array have some discrepancies? Notably the hostname?

I tend to worry about my daters incessantly...

On Thu, Jun 3, 2010 at 4:42 AM, Christian Gatzemeier
<c.gatzemeier@tu-bs.de> wrote:
> Andrew Dunn <andrew.g.dunn.dod <at> gmail.com> writes:
>
>> What is the meaning of Preferred Minor : 2 ?
>
> Most of the time that array will get assembled as md2, but conflicts,
> other numbers and errors stemming from differently assembled arrays can
> occur.
>
> From https://wiki.ubuntu.com/ReliableRaid:
> "Mdadm is setting up arrays according to unreliable superblock
> information. (Device "minor" numbers, labels and hostnames in
> superblocks are not sure to be unique and can be outdated.) This is
> combined with the idea of fixing the unreliability by limiting array
> assembly with information from mdadm.conf. (Defining PARTITIONS, ARRAY,
> HOMEHOST lines.) Consequently this forces setup tools, admins and
> installers to create mdadm.conf files and subjects them to the exact
> same reliability problems."
>
> I guess this conflict prone behavior is mostly legacy behavior from the
> past, or is there still an apparent reason to expose dynamic kernel
> internals like the minor to userspace instead of basing device node name on
> "md-UUID"?
>
> Regards,
> Christian
>
>
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> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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>
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