var warn = 20
var crit = 5

stream
    |from()
        .measurement('disk')
        .groupBy('host', 'path')
    |eval(lambda: 100 - int("used_percent"))
        .as('free_space')
    |log()
      .level('WARN')

    |alert()
        .warn(lambda: "free_space" < warn)
        .crit(lambda: "free_space" < crit)
        .id('disk_space :: {{ index .Tags "host"}}')
        .message(
'[{{ .Level }}] Low free disk space
Service: {{ index .Tags "service"}}
Stack: {{ index .Tags "stack"}}
Host: {{ index .Tags "host"}}
Volume: {{ index .Tags "path"}}
Free space: {{ index .Fields "free_space"}}%'
        )
        .opsGenie()