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 ### CloudWatch Tips
 
 * Some very common use cases for CloudWatch are **[billing alarms](http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/monitor_estimated_charges_with_cloudwatch.html)**, **instance** **or [load balancer up/down alarms](http://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-cloudwatch-metrics.html)**, and **disk usage alerts**.
-* You can use [EC2Config](http://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/UsingConfig_WinAMI.html#send_logs_to_cwl) to monitor watch memory and disk metrics on Windows platform instances.
+* You can use [EC2Config](http://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/UsingConfig_WinAMI.html#send_logs_to_cwl) to monitor watch memory and disk metrics on Windows platform instances. For Linux, there are [example scripts](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/mon-scripts.html) that do the same thing.
 * You can [publish your own metrics](http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/publishingMetrics.html) using the AWS API. [Incurs additional cost](https://aws.amazon.com/cloudwatch/pricing/).
 * You can stream directly from CloudWatch Logs to a Lambda or ElasticSearch cluster by creating [subscriptions](http://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/Subscriptions.html) on Log Groups.
 * Don't forget to take advantage of the [CloudWatch non-expiring free tier](https://aws.amazon.com/free/#Amazon_CloudWatch).
 
 ### CloudWatch Gotchas and Limitations
 
+* 🔸Metrics in CloudWatch originate [on the hypervisor](https://forums.aws.amazon.com/message.jspa?messageID=403578). The hypervisor doesn't have access to OS information, so certain metrics (most notably memory utilization) are not available unless pushed to CloudWatch from inside the instance.
 * 🔸You can not use [more than one metric for an alarm](https://forums.aws.amazon.com/thread.jspa?threadID=94984).
 * 🔸Notifications you receive from alarms will not have any contextual detail; they have only the specifics of the threshold, alarm state, and timing.
+* 🔸Minimum granularity in CloudWatch is 1 minute. That means that multiple values of a metric that are pushed to CloudWatch within the same minute are aggregated into minimum, maximum, average and total (sum) per minute.
+* 🔸Data about metrics is kept in CloudWatch [for 15 months](https://aws.amazon.com/blogs/aws/amazon-cloudwatch-update-extended-metrics-retention-user-interface-update/), starting November 2016 (used to be 14 days). Minimum granularity increases after 15 days.
 
 AMIs
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