Omeka has encountered an error

Zend_Db_Statement_Mysqli_Exception

Mysqli statement execute error : Disk full (/var/tmp/#sql_9fc_6); waiting for someone to free some space...

Zend_Db_Statement_Mysqli_Exception: Mysqli statement execute error : Disk full (/var/tmp/#sql_9fc_6); waiting for someone to free some space... in /var/www/html/digital/application/libraries/Zend/Db/Statement/Mysqli.php:214
Stack trace:
#0 /var/www/html/digital/application/libraries/Zend/Db/Statement.php(303): Zend_Db_Statement_Mysqli->_execute(Array)
#1 /var/www/html/digital/application/libraries/Zend/Db/Adapter/Abstract.php(480): Zend_Db_Statement->execute(Array)
#2 /var/www/html/digital/application/libraries/Zend/Db/Adapter/Abstract.php(776): Zend_Db_Adapter_Abstract->query('SELECT `element...', Array)
#3 /var/www/html/digital/application/libraries/Omeka/Db.php(79): Zend_Db_Adapter_Abstract->fetchAssoc(Object(Omeka_Db_Select))
#4 /var/www/html/digital/plugins/Reference/views/helpers/Reference.php(297): Omeka_Db->__call('fetchAssoc', Array)
#5 /var/www/html/digital/plugins/Reference/views/helpers/Reference.php(37): Reference_View_Helper_Reference->_getReferencesList(Array)
#6 /var/www/html/digital/plugins/Reference/controllers/IndexController.php(47): Reference_View_Helper_Reference->getList('type')
#7 /var/www/html/digital/application/libraries/Zend/Controller/Action.php(516): Reference_IndexController->listAction()
#8 /var/www/html/digital/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('listAction')
#9 /var/www/html/digital/application/libraries/Zend/Controller/Front.php(954): Zend_Controller_Dispatcher_Standard->dispatch(Object(Zend_Controller_Request_Http), Object(Zend_Controller_Response_Http))
#10 /var/www/html/digital/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(105): Zend_Controller_Front->dispatch()
#11 /var/www/html/digital/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#12 /var/www/html/digital/application/libraries/Omeka/Application.php(73): Zend_Application->run()
#13 /var/www/html/digital/index.php(23): Omeka_Application->run()
#14 {main}