Configuring compression

Hello, I'm using elasticsearch as a backend for graylog2. I've seen
recently that elasticsearch comes with an option that may help me with logs
since it stores each line as a documment and they are small. The option is
index.store.compress.stored.

How would I go about configuring this for an already running elasticsearch
index?
How would I go about making sure it's actually configured an working?

Thank you.

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