new perf example

Author: noakup

Log Analytics.md

@@ -34,6 +34,7 @@
 ### Perf
 ##### [Correlate computer memory and CPU](~/examples/Log Analytics/Correlate computer memory and cpu.md)
 ##### [Perf CPU Utilization graph per computer](~/examples/Log Analytics/Perf CPU Utilization graph per computer.md)
+##### [Calculate computer availability rate](~/examples/Log Analytics/Calculate computer availability rate.md)
 
 ### Alert
 ##### [Count alerts by severity per day](~/examples/Log Analytics/Count and chart alerts severity per day.md)

Log Analytics/Calculate computer availability rate.md

@@ -0,0 +1,16 @@
+## Calculate computer availability rate
+#### #bin_at #countif #hourofday
+<!-- article_id: 1601‎201803827040 -->
+
+For each computer, calculate the availability rate calculated since midnight. Availability is defined as "at least 1 heartbeat per hour".
+
+```OQL
+let midnight=startofday(now());
+Heartbeat
+| where TimeGenerated>midnight
+| summarize heartbeat_per_hour=count() by bin_at(TimeGenerated, 1h, midnight), Computer
+| extend available_per_hour=iff(heartbeat_per_hour>0, true, false)
+| summarize total_available_hours=countif(available_per_hour==true) by Computer 
+| extend number_of_buckets=hourofday(now())+1
+| extend availability_rate=total_available_hours*100/number_of_buckets
+```