What is the state of my destination index after ElasticSearch's reindexing api completes?

Question

I needed to change the mappings in a fairly large index, so I created a new index with good mappings and I am using the reindexing api to move data from my old index to the new one. This process is going to last about 18 hours so what is my destination index going to look like? The source index at the point the command was started or is it smart enough to catch me up to the source index at the moment it finishes?

If the answer is the former(the state of the source when the task started) is there a simple way to get it to pick up everything that was created during the 18 hours when the task was running?

Answer 1

Only way to be sure is to stop writing to the source index until reindexing is complete. BTW 18 hours sound like a lot, why does it take so long ? If your source & destination index has same number of shards, you can try taking a snapshot of source index and restore it to your destination index using rename_pattern and rename_replacement options as described here . snapshot & restore should be lot faster than 18 hours