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Tuning your cluster for indexing speed

The following configurations demonstrated an improvement in throughput of around 60% when running an indexing-only workload as compared to the out-of-the-box experience. The workload did not incorporate search or other scenarios. Only the OpenSearch server process was run on the machines, with the benchmark clients hosted on a different node.

The execution environment was comprised of Intel EC2 instances (r7iz.2xlarge) in the AWS Cloud, and the workload used was the StackOverflow dataset available as part of OpenSearch Benchmark.

Java heap size

A larger Java heap size is useful for indexing. Setting the Java min and max heap sizes to 50% of the RAM size shows better indexing performance on EC2 instances.

Flush translog threshold

The default value for flush_threshold_size is 512 MB. This means that the translog is flushed when it reaches 512 MB. The weight of the indexing load determines the frequency of the translog. When you increase index.translog.flush_threshold_size, the node performs the translog operation less frequently. Because flushes are resource-intensive operations, reducing the frequency of translogs improves indexing performance. By increasing the flush threshold size, the OpenSearch cluster also creates fewer large segments instead of multiple small segments. Large segments merge less often, and more threads are used for indexing instead of merging.

For pure indexing workloads, consider increasing the flush_threshold_size to 25% of the Java heap size, for example, to improve indexing performance.

An increased index.translog.flush_threshold_size can also increase the time that it takes for a translog to complete. If a shard fails, then recovery takes more time because the translog is larger.

Before increasing index.translog.flush_threshold_size, call the following API operation to get current flush operation statistics:

curl -XPOST "os-endpoint/index-name/_stats/flush?pretty"

Replace the os-endpoint and index-name with your endpoint and index name.

In the output, note the number of flushes and the total time. The following example output shows that there are 124 flushes, which took 17,690 milliseconds:

     "flush": {
          "total": 124,
          "total_time_in_millis": 17690

To increase the flush threshold size, call the following API operation:

curl -XPUT "os-endpoint/index-name/_settings?pretty" -d "{"index":{"translog.flush_threshold_size" : "1024MB"}}"

In this example, the flush threshold size is set to 1024 MB, which is ideal for instances that have more than 32 GB of memory.

Choose the appropriate threshold size for your cluster.

Run the stats API operation again to see whether the flush activity changed:

curl -XGET "os-endpoint/index-name/_stats/flush?pretty"

It’s a best practice to increase the index.translog.flush_threshold_size only for the current index. After you confirm the outcome, apply the changes to the index template.

Index refresh interval

By default, OpenSearch refreshes indexes every second. OpenSearch only refreshes indexes that have received at least one search request in the last 30 seconds.

When you increase the refresh interval, the data node makes fewer API calls. To prevent 429 errors, it’s a best practice to increase the refresh interval.

If your application can tolerate increasing the amount of time between when a document is indexed and when it becomes visible, you can increase the index.refresh_interval to a larger value, for example, 30s, or even disable it in a pure indexing scenario in order to improve indexing speed.

Index buffer size

If the node is performing heavy indexing, ensure that the index buffer size is large enough. You can set the index buffer size to be either a percentage of the Java heap size or the number of bytes. In most cases, the default value of 10% of JVM memory is sufficient. You can try increasing it to up to 25% for further improvement.

Concurrent merges

The maximum number of concurrent merges is specified as max_merge_count. The concurrentMergeScheduler controls the execution of merge operations when they are needed. Merges run in separate threads, and when the maximum number of threads is reached, further merges will wait until a merge thread becomes available. In cases where index throttling is an issue, consider increasing the number of merge threads beyond the default value.

Shard distribution

To ensure that the shards are distributed evenly across the data nodes of the index into which you’re ingesting, use the following formula to confirm that the shards are evenly distributed:

Number of shards for index = k * (Number of data nodes), where k is the number of shards per node

For example, if there are 24 shards in the index, and there are 8 data nodes, then OpenSearch assigns 3 shards to each node.

Setting replica count to zero

If you anticipate heavy indexing, consider setting the index.number_of_replicas value to 0. Each replica duplicates the indexing process. As a result, disabling the replicas improves your cluster performance. After the heavy indexing is complete, reactivate the replicated indexes.

If a node fails while replicas are disabled, you might lose data. Disable the replicas only if you can tolerate data loss for a short duration.

Experiment to find the optimal bulk request size

Start with a bulk request size of 5 MiB to 15 MiB. Then slowly increase the request size until the indexing performance stops improving.

Use an instance type that has SSD instance store volumes (such as I3)

I3 instances provide fast and local memory express (NVMe) storage. I3 instances deliver better ingestion performance than instances that use General Purpose SSD (gp2) Amazon Elastic Block Store (Amazon EBS) volumes. For more information, see Petabyte scale for Amazon OpenSearch Service.

Reduce response size

To reduce the size of the OpenSearch response, use the filter_path parameter to exclude unnecessary fields. Be sure that you don’t filter out any fields that are required for identifying or retrying failed requests. These fields can vary by client.

In the following example, the index-name, type-name, and took fields are excluded from the response:

curl -XPOST "es-endpoint/index-name/type-name/_bulk?pretty&filter_path=-took,-items.index._index,-items.index._type" -H 'Content-Type: application/json' -d'
{ "index" : { "_index" : "test2", "_id" : "1" } }
{ "user" : "testuser" }
{ "update" : {"_id" : "1", "_index" : "test2"} }
{ "doc" : {"user" : "example"} }

Compression codecs

In OpenSearch 2.9 and later, there are two new codecs for compression: zstd and zstd_no_dict. You can optionally specify a compression level for these in the index.codec.compression_level setting with values in the [1, 6] range. Benchmark data shows that zstd provides a 7% better write throughput and zstd_no_dict provides a 14% better throughput, along with a 30% improvement in storage compared with the default codec. For more information about compression, see Index codecs.