monitoring: Fix "10% OSDs down" alert description

The alert was triggered when less than 90% of OSDs were _up_, but then the
description took that value and described it as the percentage of OSDs being
_down_. So with 12% of OSDs down, the alert description would read:

```
88% or 88 of 100 OSDs are down (>=10%).
```

which can be panic-inducing.

This commit changes the alert expression to actually compute the ratio of OSDs
being down, which makes the correct value appear in the description.

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>
(cherry picked from commit 653c3f66823179fc5b9cbb74ff932d61a6c4178c)

diff --git a/monitoring/prometheus/alerts/ceph_default_alerts.yml b/monitoring/prometheus/alerts/ceph_default_alerts.yml
index aebd816156066196da0b4eec3374ceab36d0fe89..acafdd6d8d96b473b5b2018e8e157562eb76eb81 100644 (file)
--- a/monitoring/prometheus/alerts/ceph_default_alerts.yml
+++ b/monitoring/prometheus/alerts/ceph_default_alerts.yml
@@ -47,14 +47,14 @@ groups:
   - name: osd
     rules:
       - alert: 10% OSDs down
-        expr: (sum(ceph_osd_up) / count(ceph_osd_up)) * 100 <= 90
+        expr: count(ceph_osd_up == 0) / count(ceph_osd_up) * 100 >= 10
         labels:
           severity: critical
           type: ceph_default
           oid: 1.3.6.1.4.1.50495.15.1.2.4.1
         annotations:
           description: |
-            {{ $value | humanize}}% or {{with query "sum(ceph_osd_up)" }}{{ . | first | value }}{{ end }} of {{ with query "count(ceph_osd_up)"}}{{. | first | value }}{{ end }} OSDs are down (>=10%).
+            {{ $value | humanize }}% or {{ with query "count(ceph_osd_up == 0)" }}{{ . | first | value }}{{ end }} of {{ with query "count(ceph_osd_up)" }}{{ . | first | value }}{{ end }} OSDs are down (≥ 10%).
 
             The following OSDs are down:
             {{- range query "(ceph_osd_up * on(ceph_daemon) group_left(hostname) ceph_osd_metadata) == 0" }}