Setting up disk space utilization alerts in Google Stackdriver is almost unbelievably tricky. It’s easy to think that you can just choose the metric “Disk Utilization” and add a condition to alert when disk utilization crosses some threshold for some length of time (such as over 55% for 1 hour).

File System Type Filter

Your first problem is that a Linux system has many types of file systems, even if it only has one disk. You probably want to alert when your root disk is running out of space; therefore, you need to filter out non-disk filesystems like procfs or tmpfs. Add a device filter to select exactly which devices you want to alert on; in my case, with only one disk per server, I match only on device rootfs.