[CentOS] // RESEND // 7.6: Software RAID1 fails the only meaningful test

(Resend: message didn't show, was my original message too big? Posted one of 
the output files to a website to see)

The point of RAID1 is to allow for continued uptime in a failure scenario. 
When I assemble servers with RAID1, I set up two HDDs to mirror each other, 
and test by booting from each drive individually to verify that it works. For 
the OS partitions, I use simple partitions and ext4 so it's as simple as 
possible.

Using the CentOS 7.6 installer (v 1810) I cannot get this test to pass in any 
way, with or without LVM. Using an older installer, it works fine (v 1611) and 
I am able to boot from either drive but as soon as I do a yum update then it 
fails. 

I think this may be related or the same issue reported in "LVM failure after 
CentOS 7.6 upgrade" since that also involves booting from a degraded RAID1 
array. 

This is a terrible bug. 

See below for some (hopefully) useful output while in recovery mode after a 
failed boot. 

### output of fdisk -l 

Disk /dev/sda: 500.1 GB, 500107862016 bytes, 976773168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk label type: dos
Disk identifier: 0x000c1fd0

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048   629409791   314703872   fd  Linux raid autodetect
/dev/sda2   *   629409792   839256063   104923136   fd  Linux raid autodetect
/dev/sda3       839256064   944179199    52461568   fd  Linux raid autodetect
/dev/sda4       944179200   976773119    16296960    5  Extended
/dev/sda5       944181248   975654911    15736832   fd  Linux raid autodetect


### output of cat /prod/mdstat 
Personalities : 
md126 : inactive sda5[0](S)
      15727616 blocks super 1.2
       
md127 : inactive sda2[0](S)
      104856576 blocks super 1.2
       
unused devices: <none>


### content of rdosreport.txt 
It's big; see 
http://chico.benjamindsmith.com/rdsosreport.txt