[systemd-devel] Errorneous detection of degraded array

[NeilBrown]

On Fri, Jan 27 2017, Andrei Borzenkov wrote:

> 26.01.2017 21:02, Luke Pyzowski пишет:
>> Hello,
>> I have a large RAID6 device with 24 local drives on CentOS7.3. Randomly (around 50% of the time) systemd will unmount my RAID device thinking it is degraded after the mdadm-last-resort at .timer expires, however the device is working normally by all accounts, and I can immediately mount it manually upon boot completion. In the logs below /share is the RAID device. I can increase the timer in /usr/lib/systemd/system/mdadm-last-resort at .timer from 30 to 60 seconds, but this problem can randomly still occur.
>> 
>> systemd[1]: Created slice system-mdadm\x2dlast\x2dresort.slice.
>> systemd[1]: Starting system-mdadm\x2dlast\x2dresort.slice.
>> systemd[1]: Starting Activate md array even though degraded...
>> systemd[1]: Stopped target Local File Systems.
>> systemd[1]: Stopping Local File Systems.
>> systemd[1]: Unmounting /share...
>> systemd[1]: Stopped (with error) /dev/md0.

This line perplexes me.

The last-resort.service (and .timer) files have a Conflict= directive
against sys-devices-virtual-block-md$DEV.device 
Normally a Conflicts= directive means that if this service starts, that
one is stopped, and if that one starts, this is stopped.
However .device units cannot be stopped:

$ systemctl show sys-devices-virtual-block-md0.device | grep Can
CanStart=no
CanStop=no
CanReload=no
CanIsolate=no

so presumable the attempt to stop the device fails, so the Conflict=
dependency cannot be met, so the last-resort service (or timer) doesn't
get started.
At least, that is what I see happening in my tests.

But your log doesn't mention sys-devices-virtual-block-md0, it
mentions /dev/md0.
How does systemd know about /dev/md0, or the connection it has with
sys-devices-virtual-block-md0 ??

Does
  systemctl  list-dependencies  sys-devices-virtual-block-md0.device

report anything interesting?  I get

sys-devices-virtual-block-md0.device
● └─mdmonitor.service

NeilBrown
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