Elasticsearch OpenSearch Flush, Translog and Refresh

By Opster Team

Updated: Mar 29, 2023

| 1 min read

In addition to reading this guide, we recommend you run the Elasticsearch Health Check-Up. It will detect issues and improve your Elasticsearch performance by analyzing your shard sizes, threadpools, memory, snapshots, disk watermarks and more.

The Elasticsearch Check-Up is free and requires no installation.

Before you begin reading this guide, we recommend you try running the OpenSearch Error Check-Up which analyzes 2 JSON files to detect many configuration errors.

To easily resolve issues in your deployment and locate their root cause, try AutoOps for OpenSearch. It diagnoses problems by analyzing hundreds of metrics collected by a lightweight agent and offers guidance for resolving them.


What is an OpenSearch flush?

In OpenSearch, flushing is the process of permanently storing data onto the disk for all of the operations that have temporarily been stored in memory. This is also known as a Lucene commit.

How are OpenSearch documents indexed?

To understand the relevance of flushing, it is necessary to understand how OpenSearch indexes documents.

As new documents are indexed, the operations are recorded on disk in the translog and stored in memory in a buffer. When the index refreshes, the documents in the buffer are added to a new Lucene segment which is also held in memory.  

Flushing is the process that stores the in-memory segments onto disk, and simultaneously closes the translog generation and starts with a new blank translog generation.

Flushing is a process which is carried out behind the scenes by OpenSearch using heuristics to optimise the tradeoffs between memory usage and disk writes.  andFlushing normally requires no intervention by users, although a flush api is available.

POST /my-index-000001/_flush

In the event of a node crashing or restarting, then OpenSearch will retrieve and flush any operations that were stored in the translog prior to the crash in order to ensure that data is not lost.

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