Understanding differences in Monitoring Dashboard Disk Space usage and Filesystem disk space usage reports
24 November 2015 02:47 PM
MarkLogic server monitoring dashboard provides a way to Monitor Disk Usage which is a key monitoring metric. Comparing the disk usage shown on monitoring dashboard with Disk space on filesystem (for example, using df –h) reveals difference between two. This article talks about these differences and reasons behind them.
To understand how to use Monitoring dashboard Disk Usage, see our documentation at https://docs.marklogic.com/guide/monitoring/dashboard#id_60621
If you add all disk usage metrics (Fast Data, Large Data, Forest Data, Forest reserve, Free) and compare it with space on your disk (using df -h or other commands) you will see a difference between those two values.
This difference exists mainly because of two reasons:
Consider below example for a host running RHEL 6 with 100GB disk space on filesystem for one database and one forest.
On the other hand, consider disk usage as shown by df -h command for filesystem:
Filesystem Size Used Avail Use% Mounted on
Adding 5% default OS reserve for Linux gives us total size for this filesystem which is more than 99GB i.e,100 GB appx.
NOTE1: The actual disk usage for forests (including Journal sizes) can be confirmed by checking the output of below command from the file system,
du -h /MarkLogic_Data/Forests/*
NOTE2: df command is using -h flag (for human readable format)
The reason for difference in metric on Monitoring dashboard and disk usage for filesystem is because monitoring history do not show Journal size and OS reserve space in the report.