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/Omeka/Db.php(79): Zend_Db_Adapter_Abstract->query('SELECT `element...')
#3 /var/www/html/digital/plugins/Reference/views/helpers/Reference.php(342): Omeka_Db->__call('query', Array)
#4 /var/www/html/digital/plugins/Reference/views/helpers/Reference.php(68): Reference_View_Helper_Reference->_count(Array)
#5 /var/www/html/digital/plugins/Reference/views/public/index/browse.php(25): Reference_View_Helper_Reference->count('subject')
#6 /var/www/html/digital/application/libraries/Omeka/View.php(114): include('/var/www/html/d...')
#7 /var/www/html/digital/application/libraries/Zend/View/Abstract.php(888): Omeka_View->_run('/var/www/html/d...')
#8 /var/www/html/digital/application/libraries/Zend/Controller/Action/Helper/ViewRenderer.php(912): Zend_View_Abstract->render(NULL)
#9 /var/www/html/digital/application/libraries/Zend/Controller/Action/Helper/ViewRenderer.php(933): Zend_Controller_Action_Helper_ViewRenderer->renderScript('index/browse.ph...', NULL)
#10 /var/www/html/digital/application/libraries/Zend/Controller/Action/Helper/ViewRenderer.php(972): Zend_Controller_Action_Helper_ViewRenderer->render()
#11 /var/www/html/digital/application/libraries/Zend/Controller/Action/HelperBroker.php(277): Zend_Controller_Action_Helper_ViewRenderer->postDispatch()
#12 /var/www/html/digital/application/libraries/Zend/Controller/Action.php(527): Zend_Controller_Action_HelperBroker->notifyPostDispatch()
#13 /var/www/html/digital/application/libraries/Zend/Controller/Dispatcher/Standard.php(308): Zend_Controller_Action->dispatch('browseAction')
#14 /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))
#15 /var/www/html/digital/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(105): Zend_Controller_Front->dispatch()
#16 /var/www/html/digital/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#17 /var/www/html/digital/application/libraries/Omeka/Application.php(73): Zend_Application->run()
#18 /var/www/html/digital/index.php(23): Omeka_Application->run()
#19 {main}