error data message when I go to view my real time visitors, says that my memory is full? (no replies)

There is an error. Please report the message (Piwik 2.1.0) and full backtrace in the Piwik forums (please do a Search first as it might have been reported already!).

Warning: session_write_close(): write failed: No space left on device (28) in /var/www/piwik/libs/Zend/Session.php on line 691

Backtrace -->

#0 Piwik\Error::errorHandler(...) called at [:]
#1 session_write_close(...) called at [/var/www/piwik/libs/Zend/Session.php:691]
#2 Zend_Session::writeClose(...) called at [:]


There is an error. Please report the message (Piwik 2.1.0) and full backtrace in the Piwik forums (please do a Search first as it might have been reported already!).

Warning: session_write_close(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/www/piwik/tmp/sessions) in /var/www/piwik/libs/Zend/Session.php on line 691

Backtrace -->

#0 Piwik\Error::errorHandler(...) called at [:]
#1 session_write_close(...) called at [/var/www/piwik/libs/Zend/Session.php:691]
#2 Zend_Session::writeClose(...) called at [:]



I have no clue what any of this means so please explain how to fix in the lamest terms possible lol. Thanks