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Re: what does table_open_cache_overflow=0 means?
Posted by: Rick James
Date: December 31, 2014 04:59PM

Perhaps you mean
Table_open_cache_overflows
(plural)?
I read that to mean that the table_open_cache has not overflown since MySQL started up. That sounds good; nothing to tune.

But, to pursue it further...
What version are you running?
SHOW VARIABLES LIKE 'table%';
SHOW GLOBAL STATUS LIKE '%open%';

----- 2012-08-07 5.6.6 Milestone 9 -- Functionality Added or Changed -- -----

To improve scalability by reducing contention among sessions for the global lock on the open tables cache, the cache now can be partitioned into several smaller cache instances. A session now need lock only one instance to access it for DML statements. This segments cache access among instances, permitting higher performance for operations that need to use the cache when many there are many sessions accessing tables. (DDL statements still require a lock on the entire cache, but such statements are much less frequent than DML statements.)

A new system variable, table_open_cache_instances ( http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_open_cache_instances ), permits control over the number of cache instances. Each instance has a size of table_open_cache ( http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_open_cache ) / table_open_cache_instances ( http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_open_cache_instances ). By default, the number of instances is 1.

Three new status variables provide information about the operation of the open tables cache. Table_open_cache_hits ( http://dev.mysql.com/doc/refman/5.6/en/server-status-variables.html#statvar_Table_open_cache_hits ) and Table_open_cache_misses ( http://dev.mysql.com/doc/refman/5.6/en/server-status-variables.html#statvar_Table_open_cache_misses ) indicate the number of hits and misses or lookups in the cache. Table_open_cache_overflows ( http://dev.mysql.com/doc/refman/5.6/en/server-status-variables.html#statvar_Table_open_cache_overflows ) indicates how many times, after a table is opened or closed, an instance has an unused entry and the size of the instance is larger than table_open_cache ( http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_open_cache ) / table_open_cache_instances ( http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_table_open_cache_instances ).

(I learned something -- These are new settings that snuck in when I was not looking.)

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Re: what does table_open_cache_overflow=0 means?
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December 31, 2014 04:59PM


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