Partition folder snapshot use du to calculate folder size and return value in Mb.
This snapshot is enabled by default and will calculate size of each partition every 24hours.
Partition with size < 1Mb are not are not reported (considered as empty).
The behavior can be changed using some configurations in slapos.cfg:

[collect]
report_disk_usage = True

disk_snapshot_process_pid_foder = /srv/slapgrid/var/run

disk_snapshot_time_cycle = 86400