Re: md prefered minor has been renumbered

[Wols Lists <antlists@youngman.org.uk>]

On 28/11/16 17:26, Randall C. Grimshaw wrote:
> I have made the mistake of booting a centos-6.8 live cd to manipulate a centos 5 system.
> as a result md5 has become md122, md2 has become md125, md4 has become md126, md0 has become md127, and most unfortunately md3 has also become md122.
> 
> smartctl shows that the WD brand drives do support SCT
> mdadm --examine was used to reveal the mish-mash using uuid numbers compared with the file /etc/mdadm.conf also referencing /etc/fstab.
> 
> can someone kindly tell me the mdadm command to put the correct numbers back.

In My Experience (I'll probably get told I'm wrong :-) mdadm.conf seems
to be pretty much ignored by modern systems. And md numbers now by
default count down from 127, not up from 0.

I think if you reboot back in to CentOS 5, you'll get the right numbers
back...
> 
> sda7,sdb7 (122) /dev/md5 uuid=11b7e8b7:ca181503:cf45af46:db2a5d9d /
> sda1,sdb1 (127) /dev/md0 uuid=d9a5774d:d930fbd8:cf45af46:db2a5d9d /boot
> sda3,sdb2 (125) /dev/md2 uuid=23a4116f:1d43390b:cf45af46:db2a5d9d /tmp
> sda5,sdb5 (122) /dev/md3 uuid=c89f6357:f1c7a0ef:cf45af46:db2a5d9d /var
> sda6,sdb6 (126) /dev/md4 uuid=67ad84bc:05153ebe:cf45af46:db2a5d9d /var/lib/mysql/cmdaemon_mon
> sda2,sdb2 (   1) /dev/md1 uuid=5d9f0b00:24d890eb:cf45af46:db2a5d9d swap
> sdc1                                                            /home
> 
That said, you should not be using fixed numbers any more - the linux
kernel does not guarantee discovery order or device names. It may
shuffle sda, sdb, sdc etc. It definitely shuffles mdnnn (my other gentoo
system causes me grief with this :-) because it's broken in other ways
too...)

What do you need your mdN names to be constant for? If you're just doing
emergency maintenance, then you might be better off using a rescue CD,
or just manually mounting those partitions for maintenance.

And if you're planning on upgrading, either convert the arrays to named
arrays, or refer to them by UUID.

Cheers,
Wol