[CentOS] LVM failure after CentOS 7.6 upgrade -- possible corruption

Wed Dec 5 17:27:22 UTC 2018
Gordon Messmer <gordon.messmer at gmail.com>

I've started updating systems to CentOS 7.6, and so far I have one failure.

This system has two peculiarities which might have triggered the 
problem.  The first is that one of the software RAID arrays on this 
system is degraded.  While troubleshooting the problem, I saw similar 
error messages mentioned in bug reports indicating that sGNU/Linux 
ystems would not boot with degraded software RAID arrays.  The other 
peculiar aspect is that the system uses dm-cache.

Logs from some of the early failed boots are not available, but before I 
completely fixed the problem, I was able to bring the system up once, 
and captured logs which look substantially similar to the initial boot. 
The content of /var/log/messages is here:

The output of lsblk (minus some VM logical volumes) is here:

As best I can tell, the LVM tools were treating software RAID component 
devices as PVs, and detecting a conflict between those and the assembled 
RAID volume.  When running "pvs" on the broken system, no RAID volumes 
were listed, only component devices.  At the moment, I don't know if the 
LVs that were activated by the initrd were backed by component devices 
or the RAID devices, so it's possible that this bug might corrupt 
software RAID arrays.

In order to correct the problem, I had to add a global_filter to 
/etc/lvm/lvm.conf and rebuild the initrd (dracut -f):
	global_filter = [ "r|vm_.*_data|", "a|sdd1|", "r|sd..|" ]

This filter excludes the LVs that contain VM data, accepts "/dev/sdd1" 
which is the dm-cache device, and rejects all other partitions on 
SCSI(SATA) device nodes, as all of those are RAID component devices.

I'm still working on the details of the problem, but I wanted to share 
what I know now in case anyone else might be affected.

After updating, look at the output of "pvs" if you use LVM on software RAID.