Open Stack

Hi All,
I have a multi node setup of openstack juno on centos 7. After I 
instantiate multiple VMs and check the nova-compute.log on compute nodes 
it shows a negative value for free disks even though the the physical 
system has a lot of free memory

on the physical system. df -h

Filesystem               Size  Used Avail Use% Mounted on
/dev/mapper/centos-root   50G  2.0G   49G   4% /
devtmpfs                  16G     0   16G   0% /dev
tmpfs                     16G     0   16G   0% /dev/shm
tmpfs                     16G  281M   16G   2% /run
tmpfs                     16G     0   16G   0% /sys/fs/cgroup
/dev/mapper/centos-home  865G   33M  865G   1% /home
/dev/sda1                494M  137M  358M  28% /boot

nova-compute.log on compute node:

2015-01-06 11:00:28.756 8123 AUDIT nova.compute.resource_tracker [-] 
Total physical ram (MB): 31911, total allocated virtual ram (MB): 17920
2015-01-06 11:00:28.756 8123 AUDIT nova.compute.resource_tracker [-] 
Free disk (GB): -113
2015-01-06 11:00:28.757 8123 AUDIT nova.compute.resource_tracker [-] 
Total usable vcpus: 16, total allocated vcpus: 10

What is the issue and how can I resolve it? Please help.

Thanks,

Priyanka