The size of a database update cycle is monitored item 86400. One day, using the latest value minus the value of the last change to show the value of 10g-0 has been to change the value of 10g.
In the update time of the day when prevvalue is 0, the previous value can not be displayed
when the update cycle after it piecemeal, prevvalue have value. Changes in the value became + / - mb
Another, zabbix from 1-86400 update cycle is
estimated to be at a point in time 86400, prevvalue is a zero