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Posted by Frank Speiser on December 13 2004 8:49pm[Delete] [Edit]

The optimal value of Qcache_lowmem_prunes is 0. You want to make sure you set the query_cache_size high enough to hold your frequently used queries. MySQL trims what is held in the query_cache when it needs space. Having said that, a total of zero Qcache_lowmem_prunes might not be possible, so you should strive to keep it as low as possible. See: query_cache_size and of course query_cache_type (should be set to 1).

Posted by Vlatko Å urlan on July 5 2010 8:33am[Delete] [Edit]

But you do not want to have MySQL spend a bunch of time sorting the cache index. Apparently there are some production case reports where too big caches caused MySQL to spend over 70% of it's processing time doing just that - sorting it's cache index. Ideally you want queries worth caching in the cache and the rest ignored. Here is a nice little treaty on a few nasty gotchas and tricks that can shave a few hours off of your 'what the heck is going on here' time: