Comment 1 for bug 1642941

Fuel uses libvirt_mof_limit parameter to configure a limit for
number of open files. This parameter defaults to 102400. A
/etc/init/libvirtd.conf file is used to set this limit on
compute nodes.

Each connection from RBD image to OSD needs 1 file descriptor (FD)
and 2 threads. An extreme case requeres connection to all OSDs in the
cluster for every OSD. For example, if you have 50 OSDs and 4096 RBDs,
than default limit will not work for you.

On the other hand, it doesn't look like a common problem: one
shouldn't use ceph to host 4096 RBDs, an RBD wouldn't necessary
require a connection to all OSDs, an OSD cluster wouldn't necessary
have such amount of OSDs.

IMO this should be considered as a known issue.