If you are experiencing MySQL startup failure on BtPanel, you are not alone. This issue is often caused by the query_cache_type and query_cache_size parameters in the BtPanel MySQL management panel. MySQL 5.6 has query cache disabled by default, but it is still supported for configuration. MySQL 5.7 also supports query cache, but it is disabled by default. However, with MySQL 8.0, the query_cache_type and query_cache_size options have been removed, along with the entire Query Cache feature. This means that if you have applied the performance adjustments in the BtPanel MySQL management panel on your MySQL 8.x.x instance, it's likely the cause of your startup failure.
To resolve this issue, set the query_cache_size to 0 and comment out the query_cache_type configuration. You can comment out the related configuration by searching for all entries starting with 'query_cache'. Once done, restart the MySQL service.
In a nutshell, Query Cache is a mechanism that caches query results to improve read performance. However, with the advent of modern databases and other caching strategies like application-level caching, Redis, and Memcached, the limitations of Query Cache have become apparent. Query Cache relies on global locks, which lead to performance degradation under high concurrency; moreover, any write operation invalidates the cache, and frequent updates to a table render the cache ineffective.
If you upgrade to MySQL 8.0 or later and rely on Query Cache, consider using Redis, Memcached, or other in-memory databases to cache query results, or implement cache management and invalidation logic in your code. This not only improves performance but also avoids the pitfalls of Query Cache. Adopting the right indexing strategies, query rewrites, and storing specific query results in memory can also help optimize database performance, ensuring that your applications remain modern and scalable.