Hello,
I do not see anything in the cold standby backup script that would cause downtime.
What size is your Mailcow instance? Mine is around 250 GB, and when I run the backup, the script finishes in 10 minutes, since it uses an incremental backup strategy. Definitely not over an hour like you’re experiencing. How often you run this script?
My first guess after reading your post is that you’re running into I/O saturation. Check the output of the backup script — each rsync should print something like (unrelated example):
sent 218 bytes received 38 bytes 512.00 bytes/sec
total size is 1024 speedup is 4.00
If the reported speed (e.g. 512.00 bytes/sec) is very low, then your bottleneck is likely disk I/O, network, or CPU.
If speedup stays low, the incremental backup might not be working correctly. And you are transferring unnecessary data.
Also, make sure that none of your other backup routines are running in parallel. Running all 3 at once could easily overload the server.