How to increase open files limit for cinder-volume?

Issue

• How to increase the open files limit for the cinder-volume service?
• Cinder volumes consistently stuck in status when issuing many tasks at once

• Getting too many open files error in cinder logs.

  ...
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/cinder/volume/drivers/rbd.py", line 637, in delete_volume
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher     rbd_image = self.rbd.Image(client.ioctx, volume_name)
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/rbd.py", line 356, in __init__
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher     self.librbd = load_librbd()
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib/python2.7/site-packages/rbd.py", line 131, in load_librbd
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher     librbd_path = find_library('rbd')
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib64/python2.7/ctypes/util.py", line 244, in find_library
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher     return _findSoname_ldconfig(name) or _get_soname(_findLib_gcc(name))
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher   File "/usr/lib64/python2.7/ctypes/util.py", line 233, in _findSoname_ldconfig
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher     f = os.popen('/sbin/ldconfig -p 2>/dev/null')
  2016-06-02 14:51:06.768 26838 TRACE oslo_messaging.rpc.dispatcher OSError: [Errno 24] Too many open files
  ...