# Elasticsearch System Parameter for ASYNC IO

**URL:** <https://discuss.elastic.co/t/elasticsearch-system-parameter-for-async-io/284750>\
**Category:** Elasticsearch\
**Created:** [September 21, 2021, 4:31pm UTC](https://discuss.elastic.co/t/elasticsearch-system-parameter-for-async-io/284750 "2021-09-21T16:31:20Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![tusharnemade](https://avatars.discourse-cdn.com/v4/letter/t/67e7ee/32.png) [@tusharnemade](https://discuss.elastic.co/u/tusharnemade)\
**Post date:** [September 22, 2021, 2:59am UTC](https://discuss.elastic.co/t/elasticsearch-system-parameter-for-async-io/284750/3 "2021-09-22T02:59:44Z")

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Hello @DavidTurner

Reference : [ES default - async or sync](https://discuss.elastic.co/t/es-default-async-or-sync/19654)

Comment : 2

```auto
Elastic Team Member
Sep 2014
Hiya

OK I see where the confusion is coming in. I used the word asynchronously
in slightly different contexts there. I will try to reword in the
Definitive Guide.

Replication is sync by default, in other words: the primary waits for
indexing to happen on the replica before it returns to the user. That
said, lots of these processes happen at the same time, so sending the
document to the replica is asynchronous. It doesn't send a change then
wait for the response before sending the next one. This all happens in
parallel.

```

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