To make the calculation of trigger expressions, calculated/aggregate items and some macros much faster, since Zabbix 2.2 a value cache option is supported by the Zabbix server.
This in-memory cache can be used for accessing historical data, instead of making direct SQL calls to the database. If historical values are not present in the cache, the missing values are requested from the database and the cache updated accordingly.
To enable the value cache functionality, an optional ValueCacheSize parameter is supported by the Zabbix server configuration file.
Two internal items are supported for monitoring the value cache: zabbix[vcache,buffer,<mode>] and zabbix[vcache,cache,<parameter>]. See more details with internal items.