Error data message when I go to view my real time visitors, says that my memory is full?

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

please upgrade ASAP to Piwik 2.6.0-rc1 where this bug has been fixed: Index of /