[CentOS] *very* ugly mdadm issue

On Tue, Sep 2, 2014 at 1:33 PM,  <m.roth at 5-cent.us> wrote:
>
>> I haven't used raw devices as members so I'm not sure I understand the
>> scenario.   However, I thought that devices over 2TB would not auto
>> assemble so you would have to manually add the ARRAY entry for
>> /dev/md4 in /etc/mdadm.conf containing /dev/sdd  and /dev/sdc for the
>> system to recognize it at bootup.
>>
> Yeah. That was one thing I discovered. Silly me, assuming that the mdadm
> would create an entry in /etc/mdadm.conf. And this is not something I do
> more than once or twice a year, and haven't this year (we have a good
> number of Dells with a PERC 7, or then there's the JetStors....).

With devices < 2TB and MBR's, you don't need /etc/mdadm.conf - the
kernel just figures it all out at boot time, regardless of the disk
location or detection order.   I have sometimes set up single
partitions as 'broken' raids just to get that autodetect/mount effect
on boxes where the disks are moved around a lot because it worked long
before distos started mounting by unique labels or uuids. And I miss
it on big drives.

>> But sdd _should_ have the correct data - it just isn't being detected
>> as a raid member.   I think with smaller devices - or at least devices
>> with smaller partitions and FD type in the MBR it would have worked
>> automatically with the kernel autodetect.
>
> Both had a GPT on them, just no partitions. And that's the thing that
> really puzzles me - why mdadm couldn't find the RAID info on /dev/sdd,
> which *had* been just fine.
>
> Anyway, the upshot was my manager was rather annoyed - I *should* have
> pulled sdc, and put in a new one, and just let that go. I still think it
> would have failed, given the inability of mdadm to find the info on sdd.
> We wound up just remaking the RAID, and rebuilding the mirror over the
> weekend.

I think either adding the ARRAY entry in /etc/mdadm.conf and rebooting
or some invocation of mdadm could have revived /dev/md4 with /dev/sdd
(and the contents you wanted) active.

-- 
   Les Mikesell
     lesmikesell at gmail.com