Solutions

Stay on top of everything MarkLogic

Be the first to know! News, product information, and events delivered straight to your inbox.

Sign Me Up

Learn

Stay on top of everything MarkLogic

Be the first to know! News, product information, and events delivered straight to your inbox.

Sign Me Up

Community

Stay on top of everything MarkLogic

Be the first to know! News, product information, and events delivered straight to your inbox.

Sign Me Up

Company

Stay on top of everything MarkLogic

Be the first to know! News, product information, and events delivered straight to your inbox.

Sign Me Up

 
Knowledgebase:
Understanding differences in Monitoring Dashboard Disk Space usage and Filesystem disk space usage reports
24 November 2015 02:47 PM

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, and
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 6 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. Adding to this, size of Journals (around 1GB for this case) and OS reserve space  (5%) 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.

 

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)

 

Conclusion

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.

 

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/

(1 vote(s))
Helpful
Not helpful

Comments (0)