Grafana loki backups using boltdb as storage

I installed grafana loki v1.5 it yesterday using docker, it’s been running for 21 hs using boltdb. When I look into the /loki/index dir I see there are 112 files named index_2519 through index_2630.
In the configuration the schema_config.index period is set to 168h.

What is the relationship between this value and the files created?

If I want to backup the logs for a specified period of time, what files should I copy from the index and chunks dir?  

Source: StackOverflow