Unclean shutdown questions

[FlyoverCountry]

I have some dumb questions, just need it cleared up.  My UPS failed and shut off power to my server.  I had NUT configured but the UPS completely failed.  I was notified by unraid of the unclean shutdown.  A parity check was not automatically started.  I shut the server down and did a parity check later. When I started the parity check I made the mistake of selecting "write corrections to parity" when I started the parity check.  The parity check finished with 0 errors.  My questions are as follows:

 

#1:  With "0 errors" does that mean it wrote corrections that I didn't see or was notified of?  Would the messaging have been different if it found errors and corrected them?

 

#2:  I fully expected to have sync errors but it reported none.  Is this because I wasn't writing anything to the server at the time of the unexpected shutdown?

 

Diagnostics attached.  Thank you for your patience in advance.

wellserver-diagnostics-20230726-1718.zip

[JonathanM]

2 hours ago, FlyoverCountry said:

#1:  With "0 errors" does that mean it wrote corrections that I didn't see or was notified of?  Would the messaging have been different if it found errors and corrected them?

It will show an error count if there are any differences in what is read from the parity disk and what is calculated from the data drives. The messaging is currently the same whether those errors are simply noted, or corrected.

2 hours ago, FlyoverCountry said:

#2:  I fully expected to have sync errors but it reported none.  Is this because I wasn't writing anything to the server at the time of the unexpected shutdown?

Correct. Parity is kept up to date in realtime as writes are done. As long as the drive internal caches have finished their writes before power is lost, there should be no sync errors.

 

If there are writes in progress, the data drives are higher priority, so it's entirely possible that a write can be successfully completed on the data drive but the parity write has yet to be fully committed, causing a sync error, a correcting parity check will complete that process. It's also possible for a power cut to corrupt a data drive if a write in progress gets cut off at just the wrong moment. In that case parity can't help, because it will have even older data. Most of the time the file system checks can at least get back to a readable state, at the expense of a corrupted file or several.