Add warning for read-write indices in force merge documentation

[ppf2]

Running force merge against a read-write index (an index that can still have read/write activity post force-merge) can cause very large segments to be produced (>5Gb per segment), and the merge policy will never consider it for merging again until it has 75% of deleted docs. This can cause very large segments to remain in the shards.

This is not currently documented (https://www.elastic.co/guide/en/elasticsearch/reference/6.2/indices-forcemerge.html). Would like to see a warning message added to the above page to specify force merge as an API call to use only against read-only indices.

[mueedc-zz]

I'd be happy to give this a shot if no one else has claimed it

[javanna]

go for it @mueedc , looking forward to your PR ;)

[elasticmachine]

Pinging @elastic/es-core-infra

[eloj]

I know this is an old:ish issue, but the warning text merged seems misleading, at least to relatively inexperienced users like myself.

I immediately assumed that I should set index.blocks.read_only (or .read_only_allow_delete) to make the index read-only before issuing a force merge, but this then results in a 403 when trying to merge.

It was explained to me on twitter that the actual meaning is that there should be no concurrent writes, not that the index should be set to read-only.

Please consider if the warning can be rewritten to be clearer. Thank you.

[laf-rge]

I know this is an old:ish issue, but the warning text merged seems misleading, at least to relatively inexperienced users like myself.

I immediately assumed that I should set index.blocks.read_only (or .read_only_allow_delete) to make the index read-only before issuing a force merge, but this then results in a 403 when trying to merge.

It was explained to me on twitter that the actual meaning is that there should be no concurrent writes, not that the index should be set to read-only.

Please consider if the warning can be rewritten to be clearer. Thank you.

I just did the same thing.

[DaveCTurner]

Thanks for the comments @laf-rge and @eloj, I opened #41624 to discuss fixes.