Speed up building overviews on a mosaic dataset

[BenjaminShelton]

I am running some tests on building overviews to see if the process can be sped up, but get confusing results.  Using the define overviews tool in ArcGIS 10.5, I defined and built overviews for about 1,000 tiles of 6" orthoimagery.  I used the default parameters and it created over 300 overviews and took 4 hours to build.  I reduced the number of levels to 3 and increased the Overview Sampling Factor to 10.  I also increased the number of rows and columns.  This resulted in only 25 overviews, but still took 4 hours to build.  How does it take the same amount of time to create 25 overviews as it does 300?  I have tried different combinations of parameters, and it always results in a 4 hour process.  Is there any other methods for speeding up the process of building overviews?  The only thing I have found that provided any efficiency is creating them on the local hard drive as opposed to an external hard drive.

[DanPatterson_Retired]

The latter would have been a suggestion.  If the external is usb connected, usb 3 will be faster https://en.wikipedia.org/wiki/USB_3.0

[SimonWoo]

Does your machine have a lot of cores?  If so, are you using the Parallel Processing environment Setting?

http://desktop.arcgis.com/en/arcmap/latest/tools/environments/parallel-processing-factor.htm

I also agree with Dan's suggestion.  I\O tends to be a bottleneck in the process, so the faster your access to the writing location, the better the performance. 

[BenjaminShelton]

Thanks for the feedback!  I took your advice and put in the environment settings for Parallel Processing to make sure it was 100%.  It took 4 hours again still, but good to make sure it was taking advantage of all 4 cores on the machine.  I also scripted it so that it created the overviews locally on a solid state drive and then moved the overviews onto the external hard drive and repaired the paths.  So far that seems to be as efficient as I can get it.

Still not sure how creating 25 overviews takes the same amount of time as creating 300+ overviews.