How to test failover unions correctly?

My use case:

I have 2 remotes, one is google drive (1:) and one is a local disk folder on my hdds (disk:). Both contains identical data, while the disk remote is jut a bit behind.

I want to use a union remote in a way so that if anything happens with my disk/google drive, applications accessing the mount wouldn't notice, while also reading first from local disk.

[union]
type = union
remotes = disk:/backup 1:

But when trying to test the failover, it doesn't seem to work. Like if I make the google drive remote broken by using a SA file that doesn't exist, even rclone ls for example doesn't work while it should at least list the files on disk.

Is it possible to achieve what I want here?

Anyone can help me ?

I've never used the rclone version of union as I'm not sure how the fail over policy works.

If you break it, what do the logs show so we can see what is happening if it isn't working.

So, I got time to do another test today

[union]
type = union
remotes = 1: V:

Where 1: is a google drive remote, and V: is a rclone serve http remote.

I created a folder uniontest and did rclone mount union: uniontest -vvv

Then I went into the http server and blocked my IP from it.

The union was usable but very slow (probably because it was waiting for a timeout on the V: remote), but no errors on the log file

But we know the default policy is to read only from the first remote, so let's change the order of the remotes so V: comes first and restart the mount.

It still worked fine, just slow.

I still don't know how would be the behavior with other issues I can't simulate easily... like 403 errors from google, overloaded hdds on my http backend, etc but lets hope for the best.

Is there any flag to speed up the mount when one of unions is unavailable?