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Briefly, this error occurs when OpenSearch fails to write the indexing buffer for a specific shard. This could be due to insufficient disk space, high I/O operations, or a network issue. To resolve this, you can try freeing up disk space, optimizing your I/O operations, or checking your network connectivity. Additionally, ensure that your shard allocation is balanced and not overloading a single node. Lastly, consider increasing the indexing buffer size if it’s too small, but be cautious as it might consume more memory.
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This guide will help you check for common problems that cause the log ” failed to write indexing buffer for shard [{}]; ignoring ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: indices, indexing, shard.
Overview
Indexing is the process of adding documents to and updating documents on an OpenSearch index.
Examples
In its simplest form, you can index a document like this:
POST /test/_doc
{
"message": "Opster Rocks OpenSearch Management"
}This will create the index “test” (if it doesn’t already exist) and add a document with the source equal to the body of the POST call. In this case, the ID will be created automatically. If you repeat this command, a second document will be created with an identical source but a different ID.
Alternatively, you can do this:
PUT /test/_doc/1
{
"message": "Opster OpenSearch Management and Troubleshooting"
}This is almost the same, but in this case, the call sets the ID of the document to 1. If you repeat the command modifying the message, you will modify the original document, replacing the previous source with the latest source.
However note that this is NOT the same as an UPDATE operation, which is a different API and allows us to modify certain fields of the document while leaving others unchanged.
Notes and good things to know
You can set your own ID if necessary (especially if you later need to update the same ID) but this comes at a performance penalty. If you don’t need to update documents, then let OpenSearch set its own ID automatically.
If you need to index many documents at once, it is much more efficient to use the BULK API to carry out these operations with a single call.
Indexing is not an immediate automatic process. Documents will not be available for search until the index has refreshed. Refresh time by default is 1 second. Increasing this time reduces the burden on the cluster of indexing, increasing indexing speed. It is possible to modify the refresh time in the index settings.
You can apply version control by setting the version parameter (?version=3) and indicating version_type=external. By doing this OpenSearch will reject any index requests where the version specified is less than the current version. This can be useful when running distributed processes and you cannot guarantee that updated documents arrive in the correct order.
PUT test/_doc/1?version=20&version_type=external
{
"message" : "using external version the document will be modified only if version is greater than previous!"
}The process of indexing is as follows
The index request is sent to the primary shard. Once the primary shard is updated, then the replication process request will be relayed to the replica shards. The command will not return until the primary shard (at least) has been updated. For greater resilience, you can specify a minimum number of shard replicas to be available before proceeding with the operation by using the parameter ?wait_for_active_shards=2
You can also specify which specific shard the index operation is sent to by using the “routing” command. There are 2 reasons that this might be done:
- Certain OpenSearch functions (parent-child documents) that require that the parent and child documents be held on the same shard.
- Secondly, it may be possible to increase search speeds and reduce load on OpenSearch by storing similar documents together on the same shard and then specifying the routing for both indexing and searching. Although this can be done explicitly during indexing, it is not recommended. It would be preferable to set this up using the index mapping, so that the routing is determined by an ID value on the source document.
Overview
Data in an OpenSearch index can grow to massive proportions. In order to keep it manageable, it is split into a number of shards. Each OpenSearch shard is an Apache Lucene index, with each individual Lucene index containing a subset of the documents in the OpenSearch index. Splitting indices in this way keeps resource usage under control. An Apache Lucene index has a limit of 2,147,483,519 documents.
Examples
The number of shards is set when an index is created, and this number cannot be changed later without reindexing the data. When creating an index, you can set the number of shards and replicas as properties of the index using:
PUT /sensor
{
"settings" : {
"index" : {
"number_of_shards" : 6,
"number_of_replicas" : 2
}
}
}The ideal number of shards should be determined based on the amount of data in an index. Generally, an optimal shard should hold 30-50GB of data. For example, if you expect to accumulate around 300GB of application logs in a day, having around 10 shards in that index would be reasonable.
During their lifetime, shards can go through a number of states, including:
- Initializing: An initial state before the shard can be used.
- Started: A state in which the shard is active and can receive requests.
- Relocating: A state that occurs when shards are in the process of being moved to a different node. This may be necessary under certain conditions, such as when the node they are on is running out of disk space.
- Unassigned: The state of a shard that has failed to be assigned. A reason is provided when this happens. For example, if the node hosting the shard is no longer in the cluster (NODE_LEFT) or due to restoring into a closed index (EXISTING_INDEX_RESTORED).
In order to view all shards, their states, and other metadata, use the following request:
GET _cat/shards
To view shards for a specific index, append the name of the index to the URL, for example:
sensor: GET _cat/shards/sensor
This command produces output, such as in the following example. By default, the columns shown include the name of the index, the name (i.e. number) of the shard, whether it is a primary shard or a replica, its state, the number of documents, the size on disk, the IP address, and the node ID.
sensor 5 p STARTED 0 283b 127.0.0.1 ziap sensor 5 r UNASSIGNED sensor 2 p STARTED 1 3.7kb 127.0.0.1 ziap sensor 2 r UNASSIGNED sensor 3 p STARTED 3 7.2kb 127.0.0.1 ziap sensor 3 r UNASSIGNED sensor 1 p STARTED 1 3.7kb 127.0.0.1 ziap sensor 1 r UNASSIGNED sensor 4 p STARTED 2 3.8kb 127.0.0.1 ziap sensor 4 r UNASSIGNED sensor 0 p STARTED 0 283b 127.0.0.1 ziap sensor 0 r UNASSIGNED
Notes and good things to know
- Having shards that are too large is simply inefficient. Moving huge indices across machines is both a time- and labor-intensive process. First, the Lucene merges would take longer to complete and would require greater resources. Moreover, moving the shards across the nodes for rebalancing would also take longer and recovery time would be extended. Thus by splitting the data and spreading it across a number of machines, it can be kept in manageable chunks and minimize risks.
- Having the right number of shards is important for performance. It is thus wise to plan in advance. When queries are run across different shards in parallel, they execute faster than an index composed of a single shard, but only if each shard is located on a different node and there are sufficient nodes in the cluster. At the same time, however, shards consume memory and disk space, both in terms of indexed data and cluster metadata. Having too many shards can slow down queries, indexing requests, and management operations, and so maintaining the right balance is critical.
How to reduce your OpenSearch costs by optimizing your shards
Watch the video below to learn how to save money on your deployment by optimizing your shards.
Log Context
Log “failed to write indexing buffer for shard [{}]; ignoring” classname is IndexingMemoryController.java.
We extracted the following from OpenSearch source code for those seeking an in-depth context :
shard.writeIndexingBuffer();
}
@Override
public void onFailure(Exception e) {
logger.warn(() -> new ParameterizedMessage("failed to write indexing buffer for shard [{}]; ignoring"; shard.shardId()); e);
}
});
}
/** force checker to run now */
