The percent of physical writes of a key block to disk versus requests to write a key block to the key cache. The MyISAM Key cache is a data structure that contains the most-used index blocks. The key cache size is controlled using the global variable ‘key_buffer_size’. The monitor threshold is analyzed against an average of values returned in polls to the agent where the Number of Samples parameter controls the number of samples included in the calculation of the average.
An unhealthy state indicates the percentage of writes to the key cache compared to the writes to disk is low. The key cache might not have enough space to store all key blocks. The key cache is updated with a least recently used algorithm.
To address low percent of key cache hits increase key_buffer_size in the configuration file. This can be done by adding the following line along with size under the instance group options: “key_buffer_size= size”. Alternatively the server can be started with the following option: “--key_buffer_size=size”