Why is there less available RAM than expected on my MySQL server?

Background

Under some circumstances, MySQL server instances may appear to consume large quantities of memory (RAM). This condition manifests itself in the following symptoms:

• When you SSH into a running MySQL Server instance and run the "free" command to gauge RAM consumption on the server, the amount of free RAM appears low.
• If you have a "memory low" alert configured for a higher memory value such as 100,000,000 bytes (95 MB)--which was the default alert threshold for some legacy ServerTemplates--you receive email alerts indicating that "free memory is low" on the MySQL server.

Answer

By default, MySQL Server's configuration settings are preset to a high memory-consumption threshold in RightScale ServerTemplates. This, in turn, helps ensure a high level of performance for your MySQL databases.

Also, due to the way that caching is handled on Linux operating systems, the quantity of free memory on a running Linux instance at any given time after the initial boot process will typically be low. The RightScale alert system does not currently distinguish between consumed memory (actively in use by a system process) and cached memory (which the system can release whenever needed by another process). Thus, it triggers an alert when the total amount of consumed and cached memory on the server drops below the threshold configured for the "memory low" alert.

Since a low memory value does not indicate an issue on the server under these circumstances, we recommend setting your server's "memory low" alert threshold to a low value, such as 10,000,000 bytes (9.5 MB). You may also want to use a "low swap space" alert as an alternative to the "memory low" alert type, as absence of available swap space on a server is typically a more reliable indicator of a low-memory condition.