Comment 1 for bug 1741275

More recent occurrence of the issue: http://logs.openstack.org/64/519664/3/check/tempest-full/dbb3f0e/controller/logs/screen-c-sch.txt.gz#_Jan_23_18_55_00_424738

Analysis of the issue (according to logs from Cinder scheduler):

Jan 23 18:54:11.059512 - scheduler receives status update with free_capacity_gb = 9.47
Jan 23 18:54:11.169724 - filter reports that free_capacity_gb: 9.47
Jan 23 18:54:12.503192 - filter reports that free_capacity_gb: 8.47
....
Jan 23 18:54:57.731754 - filter reports that free_capacity_gb: 0.47
Jan 23 18:55:00.424738 - Insufficient free virtual space (-10.6GB)

Value -10.6 is calculated and one of ways to get it is when backend_state.free_capacity_gb = -0.53 (https://github.com/openstack/cinder/blob/master/cinder/scheduler/filters/capacity_filter.py#L74-L142)

The next status update goes at Jan 23 18:55:05.453072 and it resets the free space value back to 9.47

To sum up: between 2 updates of capacity numbers scheduler allocates a number of volumes; each allocation reduces local value of free disk space, until it gets below 0.