According to the bulletin on ABB Library/SolutionsBank 3BSE059394:
"In 800xA there is a command line tool "afwsysinfo.exe" that can be used to present the status on Aspect Directory database fragmentation. A recommendation is to perform corrective actions when the “DB Free Portion” exceeds 50%."
21% is OK.
I would also recommend to perform defragmentation of Aspect directory only if free DB portion is >50%.Fragmentation of AD can result in slow/poor performance (Operations will be slower down) of PPA depending on the disk size, disk condition and number of times disk accesses.
Note that it is recommended to defragment the Aspect Directory before defragment the hard drive that the Aspect Directory is stored on. Try to defrag AD using below procedure -
1. full system backup
2. Perform a system restore
You can first do the backup and then try to do the defrag
In your case it will be bit tricky because you have a Single node system (non redundant AS). In running plant it will definitely interrupt the plant operation activities. So you have to select a time for defrag when plant operation is least used.
Note -:Best way will be to open a Supportline case and take their guidance to defrag AD in a non redundant Aspect Servers scenario.
In case of redundant AS it is easy to defrag AD.
Let me know if this solves your query.