Description of problem:
For example, if users want ro run more than 1000 containers, they must explicitly increase max open files for running libvirt daemon rather than just modify 'max_clients' and 'max_workers' in  libvirtd.conf, it's hard to know concrete numbers for users, so we require a document to introduce it.

Version-Release number of selected component (if applicable):
# rpm -q libvirt libvirt-sandbox
libvirt-1.1.1-21.el7.x86_64
libvirt-sandbox-0.5.0-8.el7.x86_64


How reproducible:
Always

Steps to Reproduce:
 sysctl fs.file-nr
fs.file-nr = 82944	0	3245263

# ulimit -n
65536

# tail -2 /etc/libvirt/libvirtd.conf
max_clients = 10000
max_workers = 10000

# grep 'open files' /proc/`pidof libvirtd`/limits
Max open files            1024                 4096                 files

Notes, it seems we should successfully start 1024 containers, but in fact, we only started 332 containers.

# systemctl restart libvirtd.service
# for i in {1..1001}; do virt-sandbox-service create -C -u httpd.service apache$i; done
# for i in {1..1001}; do virsh -c lxc:/// start apache$i; done

Only successfully start 332 containers.

# virsh -c lxc:/// -q list|wc -l
332

# virsh -c lxc:/// start apache333
Failed to start domain apache333
annot open /dev/null: Too many open files

Actual results:
Pease see above.

Expected results:
Fix it.

Additional info:

# lsof|grep `pidof libvirtd`|wc -l
12294

Notes, in fact, there is 12294 open files.

To increase max open files for running libvirt daemon.
# prlimit --nofile=65535:65535 --pid `pidof libvirtd`

# grep 'open files' /proc/`pidof libvirtd`/limits
Max open files            65535                65535                files

Try again.
# virsh -c lxc:/// start apache333
Domain apache333 started

Now, continue to start the rest of containers.
# for i in {334..1001}; do virsh -c lxc:/// start apache$i; done

Everything is okay now.
# virsh -c lxc:/// -q list|wc -l
1001

The current open files.
# lsof|grep `pidof libvirtd`|wc -l
34345


For more, please see https://bugzilla.redhat.com/show_bug.cgi?id=980743#c4.