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The maximum size of the temporary file that MySQL is permitted to use while re-creating a MyISAM index (during REPAIR TABLE , ALTER TABLE , or LOAD DATA ). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. The value is given in bytes. 

If MyISAM index files exceed this size and disk space is available, increasing the value may help performance. The space must be available in the file system containing the directory where the original index file is located. 

• myisam_mmap_size 

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The maximum amount of memory to use for memory mapping compressed MyISAM  files. If many compressed MyISAM tables are used, the value can be decreased to reduce the likelihood of memory- swapping problems. 

• myisam_recover_options 

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