Applies to DSE Search versions 3.2 to 6.8
For some reason finding this information is dreadfully stupidly hard and I finally decided to write it down myself since I own it and someone may benefit from it some day. There are three ways that I know of (I am sure there is a JMX way I just do not happen to know it atm)
curl "http://localhost:8983/solr/admin/cores?action=STATUS&wt=json&indent=true" | jq '.status."mykeyspace.mytable".index.size'
ls -A -R -g -o "$@" /var/lib/cassandra/data/solr.data/mykeyspace.mytable/index | awk '{n1 += $3} END {print n1}' | awk '{ byte =$1 /1024/1024; print byte " MB" }'
I have had a deep dive to find this so many times I wish I had just posted this years ago