On 2/18/21 9:03 PM, Chris Murphy wrote:

Once everything else is figured out, you should consider converting
metadata to raid1c3.

Got it.

The new replacement is devid 0 during the replacement. The drive being
replaced keeps its devid until the end, and then there's a switch,
that device is removed, and the signature on the old drive is wiped.
Sooo.... something is still wrong with the above because there's no
devid 3, there's kernel and btrfs check messages saying devid 3 is
missing.

It doesn't seem likely that /dev/sdc3 is devid 3 because it can't be
both missing and be the mounted dev node.

It seems I was unclear. I removed the old drive prior to the
replacement, hence degraded mode.

A while ago, I imaged the drives, to see what I could do without risk
(on another machine). Turns out I was able to mount the filesystem using
-o ro,nologreplay,degraded and copy almost all files. A small number
were unreadable/un-stat-able. Fortunately nothing critical, though the
OS may well be unusable.

(Also, in case you were wondering, memory testing has revealed no errors
so far.)

If a tree log is damaged and prevents mount then, you need to make a
calculation. You can try to mount with ro,nologreplay and freshen
backups for anything you'd rather not lose - just in case things get
worse. And then you can zero the log and see if that'll let you
normally mount the device (i.e. rw and not degraded). But some of it
will depend on what's wrong.

That doesn't work. It gives the same errors as when I tried to run
check, but repeated once each for extent tree and device tree. It just
can't get past this problem.

At this point, I think it's best to just reinstall with a fresh
filesystem, and not make the same mistakes. Thanks for the help, once again.