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Re: Key Efficiency Graph
Posted by: Bill Karwin
Date: May 25, 2006 11:32AM

A higher hitrate in a caching system is good.

It means that requests for elements in the index are getting satisfied by the cache, instead of being required to go to the disk to load them. That's what a "hit" means for a cache: the requested data was supplied by the cache, instead of the disk.

Initially, the cache is empty, so the hit rate will be low. As indexes are loaded into the cache, subsequent requests for those indexes will be satisfied by the cache. If your cache is large enough to store all the indexes that are requested, the hit rate should reach 100%. That's good.

If your cache isn't configured large enough to hold the indexes, requests for those indexes cause parts of the index to be reloaded into the cache from disk, which cause sother parts of the index to be expelled from the cache. This would translate into a lower average hit rate, and it should be avoided.

You _can_ control the hit rate, by reducing the amount of memory allocated to key buffers until it's less than the size of your indexes! ;-) If you do this, be prepared for your disk activity to go way up, and your site's performance to go way down.

Bill K.

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Written By
May 25, 2006 05:56AM
Re: Key Efficiency Graph
May 25, 2006 11:32AM
May 25, 2006 05:55PM

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