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Knowledgebase:
Understanding differences in Monitoring Dashboard Disk Space usage and Filesystem disk space usage reports
29 October 2019 09:49 AM

Summary

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.

 

Details

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:
1. Monitoring history dashboard displays disk space usage excluding Forest journal sizes in MB & GB 
2. On Linux, by default around 5% of the filesystem is reserved for cases where the filesystem fills up to prevent serious problems and for its own purposes. For example for keeping backups of its internal data structures.

 

An example

Consider below example for a host running RHEL 7 with 100GB disk space on filesystem for one database and one forest.

Disk usage as shown by Monitoring dashboard:
Free                   92.46 GB      98.17%
Forest Reserve      1.14 GB       1.21%
Forest Data          0.57 GB        0.60%
Large Data           0.02 GB        0.02%

Total from Monitoring dashboard is around 94.19 GB. When we add the size of Journals (around 1GB for this case), and OS reserve space (5%), the total comes out to be 100GB which is total capacity of disk in this example.

 

On the other hand, consider disk usage as shown by df -h command for filesystem:

Filesystem                    Size Used Avail Use% Mounted on
/dev/mapper/Data1-Vol1 99G 2.1G 92G    3%   /myspace

Adding 5% default OS reserve for Linux gives us total size for this filesystem which is more than 99GB i.e,100 GB appx.

Items of Note

  • The Dashboard:Disk Space uses KB/MB/GB, which means 1 KB = 1000 B, not KiB/MiB/GiB where 1 KiB = 1024 B.
  • The actual disk usage for forests (including Journal sizes) can be confirmed by checking the output of below command from the file system:
    • du --si -h /MarkLogic_Data/Forests/*
      • -h flag is for human readable format
      • --si flag is for using KB/MB/GB instead of the default KiB/MiB/GiB

Conclusion

The reason for difference in metrics on Monitoring dashboard and disk usage for filesystem is because monitoring history does not show Journal size and OS reserve space in the report.

 

Useful Links:

https://docs.marklogic.com/guide/monitoring/dashboard#id_60621

http://serverfault.com/questions/315181/df-says-disk-is-full-but-it-is-not

http://www.walkernews.net/2011/01/22/why-the-linux-df-command-shows-lesser-free-disk-space/

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