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    <p>Database memory usage is a complex topic. The <a href="http://www.mysqlperformanceblog.com/2006/05/17/mysql-server-memory-usage/" rel="noreferrer">MySQL Performance Blog</a> does a good job of covering your question, and lists many reasons why it's hugely impractical to "reserve" memory.</p> <p>If you really want to impose a hard limit, you could do so, but you'd have to do it at the OS level as there is no built-in setting. In linux, you could utilize <a href="http://www.linuxhowtos.org/Tips%20and%20Tricks/ulimit.htm" rel="noreferrer">ulimit</a>, but you'd likely have to modify the way MySQL starts in order to impose this.</p> <hr> <p>The best solution is to tune your server down, so that a combination of the usual MySQL memory settings will result in generally lower memory usage by your MySQL installation. This will of course have a negative impact on the performance of your database, but some of the settings you can tweak in <code>my.ini</code> are:</p> <pre><code>key_buffer_size query_cache_size query_cache_limit table_cache max_connections tmp_table_size innodb_buffer_pool_size </code></pre> <p>I'd start there and see if you can get the results you want. There are <a href="http://dev.mysql.com/doc/refman/5.0/en/server-parameters.html" rel="noreferrer">many</a> <a href="http://www.ibm.com/developerworks/library/l-tune-lamp-3.html" rel="noreferrer">articles</a> out there about adjusting MySQL memory settings.</p> <hr> <p>Edit:</p> <p>Note that some variable names <a href="http://dev.mysql.com/doc/mysqld-version-reference/en/mysqld-version-reference-optvar-changes-5-1.html" rel="noreferrer">have changed in the newer 5.1.x releases of MySQL</a>.</p> <p>For example:</p> <pre><code>table_cache </code></pre> <p>Is now:</p> <pre><code>table_open_cache </code></pre>
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    1. POMySQL maximum memory usage
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    1. COHi! Thanks for your answer. I have noticed that the equation that people quote is the following: key_buffer_size+(read_buffer_size+sort_buffer_size)*max_connections=Total Memory. I have set the following: key_buffer_size=128M, read_buffer_size=1M, sort_buffer_size=2M, max_connections=120, and the total memory on the server is 512M. However, after many queries, the free memory has gone as low as 12M and would probably continue to go down with further usage. Is there a reason why this is so and can it be prevented? Thanks!
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    2. COOr maybe I need to take into consideration not the total memory on the server (512M) but the free memory (i.e. memory available after loading all OS related and other programs)?
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    3. COIf you're going to modify tmp_table_size with the intention of increasing the size of temp tables that can be stored in RAM, remember to also increase the max_heap_table_size - as MySQL uses the minimum of the two...
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