A badly designed cache can do more harm than good. In the worst case a cache can increase database server load instead of minimizing it. An overload situation can occur if a highly shared cache entry expires (cache stampeding).
Cache entries are shared and reused to a different degree depending on the storage used. The default storage handler stores cache entries in process memory. Thus, a cache entry can be reused for the life-span of a process. Other PHP processes cannot access it. If Memcache is used, a cache entry can be shared among multiple PHP processes and even among multiple machines, depending on the set up being used.
If a highly shared cache entry stored, for example, in Memcache expires, many clients gets a cache miss. Many client requests can no longer be served from the cache but try to run the underlying query on the database server. Until the cache entry is refreshed, more and more clients contact the database server. In the worst case, a total lost of service is the result.
The overload can be avoided using a storage handler which limits the reuse of cache entries to few clients. Then, at the average, its likely that only a limited number of clients will try to refresh a cache entry concurrently.
Additionally, the built-in slam defense mechanism can and should be used. If slam defense is activated an expired cache entry is given an extended life time. The first client getting a cache miss for the expired cache entry tries to refresh the cache entry within the extended life time. All other clients requesting the cache entry are temporarily served from the cache although the original TTL of the cache entry has expired. The other clients will not experience a cache miss before the extended life time is over.
Example #1 Enabling the slam defense mechanism
The function mysqlnd_qc_get_core_stats() returns an array of statistics. The statistics slam_stale_refresh and slam_stale_hit are incremented if slam defense takes place.
It is not possible to give a one-fits-all recommendation on the slam defense configuration. Users are advised to monitor and test their setup and derive settings accordingly.