What are solr index.properties files

[Webster Homer <we...@sial.com>]

While upgrading our QA solr 6.1 solrclouds to Solr 7.2.0 I discovered that
some of our index folders for a replica had directory names like
index.20170830071504690

These replicas also had a file index.properties which indicates which index
directory is current.

We don't see this configuration in our Dev solrclouds, nor

Why would we have these folders in solrcloud indexes?

What configuration controls this behavior?

Is it normal for solrcloud?

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