Opscenter agent fails to create the diagnostic tarball to avoid filling up the disk partition. The reason is not always clear
Typically this type of message will be seen in the opscenterd.log file
2015-07-09 10:15:54+0000 [MyCluster] ERROR: Not enough space to write the diagnostic file out on OpsCenter machine
2015-07-09 10:15:54+0000  ERROR: Failed to build diagnostic tarball:
Can't write out diagnostic file for OpsCenter machine, because it will fill up a disk Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/opscenterd/WebServer.py", line 3645, in DiagnosticDataController
This is a valid error and the likely cause is a large log file in the /var/log/cassandra/ directory or other related directories.
Check the /var/log/ directory for large log files that OpsCenter will collect that may be too large for the /tmp directory.