Briefly, this error occurs when the number of slices specified for a task in Elasticsearch exceeds the maximum limit. Slices are used to break down tasks into smaller parts for parallel processing. To resolve this, you can reduce the number of slices to a number within the acceptable limit. Alternatively, you can increase the maximum limit of slices, but this might impact the performance of your Elasticsearch cluster. Always ensure the number of slices is optimal for your specific use case and system resources.
This guide will help you check for common problems that cause the log ” The number of slices [” + numSlices + “] is too large. It must ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: search.
Overview
Search refers to the searching of documents in an index or multiple indices. The simple search is just a GET API request to the _search endpoint. The search query can either be provided in query string or through a request body.
Examples
When looking for any documents in this index, if search parameters are not provided, every document is a hit and by default 10 hits will be returned.
GET my_documents/_search
A JSON object is returned in response to a search query. A 200 response code means the request was completed successfully.
{
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 2,
"successful" : 2,
"failed" : 0
},
"hits" : {
"total" : 2,
"max_score" : 1.0,
"hits" : [
...
]
}
}Notes and good things to know
- Distributed search is challenging and every shard of the index needs to be searched for hits, and then those hits are combined into a single sorted list as a final result.
- There are two phases of search: the query phase and the fetch phase.
- In the query phase, the query is executed on each shard locally and top hits are returned to the coordinating node. The coordinating node merges the results and creates a global sorted list.
- In the fetch phase, the coordinating node brings the actual documents for those hit IDs and returns them to the requesting client.
- A coordinating node needs enough memory and CPU in order to handle the fetch phase.
Log Context
Log “The number of slices [” + numSlices + “] is too large. It must ” class name is DefaultSearchContext.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
if (sliceBuilder != null) {
int sliceLimit = indexService.getIndexSettings().getMaxSlicesPerScroll();
int numSlices = sliceBuilder.getMax();
if (numSlices > sliceLimit) {
throw new QueryPhaseExecutionException(this; "The number of slices [" + numSlices + "] is too large. It must "
+ "be less than [" + sliceLimit + "]. This limit can be set by changing the [" +
IndexSettings.MAX_SLICES_PER_SCROLL.getKey() + "] index level setting.");
}
}
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