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Briefly, this error occurs when the disk space on a node in an OpenSearch cluster falls below the “low disk watermark” threshold, but then recovers. OpenSearch uses watermarks to protect against running out of disk space. When the low watermark is exceeded, OpenSearch will stop allocating new shards to the node. To resolve this issue, you can either increase your disk space, delete unnecessary data, or adjust the watermark thresholds in the OpenSearch settings. However, be careful with the last option as it could lead to a risk of running out of disk space.
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This guide will help you check for common problems that cause the log ” low disk watermark [{}] no longer exceeded on {} ” to appear. To understand the issues related to this log, read the explanation below about the following OpenSearch concepts: allocation, routing, cluster.
Overview
In OpenSearch, routing refers to document routing. When you index a document, OpenSearch will determine which shard the document should be routed to for indexing.
The shard is selected based on the following formula:
shard = hash(_routing) % number_of_primary_shards
Where the default value of _routing is _id.
It is important to know which shard the document is routed to because OpenSearch will need to determine where to find that document later on for document retrieval requests.
Examples
In twitter index with 2 primary shards, the document with _id equal to “440” gets routed to the shard number:
shard = hash( 440 ) % 2
PUT twitter/_doc/440
{
...
}Notes and good things to know
- In order to improve search speed, you can create custom routing. For example, you can enable custom routing that will ensure that only a single shard will be queried (the shard that contains your data).
- To create custom routing in OpenSearch, you will need to configure and define that not all routing will be completed by default settings. ( v <= 5.0)
PUT my_index/customer/_mapping
{
"order":{
"_routing":{
"required":true
}
}
}- This will ensure that every document in the “customer” type must specify a custom routing.
Overview
An OpenSearch cluster consists of a number of servers (nodes) working together as one. Clustering is a technology which enables OpenSearch to scale up to hundreds of nodes that together are able to store many terabytes of data and respond coherently to large numbers of requests at the same time.
Search or indexing requests will usually be load-balanced across the OpenSearch data nodes, and the node that receives the request will relay requests to other nodes as necessary and coordinate the response back to the user.
Notes and good things to know
The key elements to clustering are:
Cluster State – Refers to information about which indices are in the cluster, their data mappings and other information that must be shared between all the nodes to ensure that all operations across the cluster are coherent.
Master Node – Each cluster must elect a single master node responsible for coordinating the cluster and ensuring that each node contains an up-to-date copy of the cluster state.
Cluster Formation – OpenSearch requires a set of configurations to determine how the cluster is formed, which nodes can join the cluster, and how the nodes collectively elect a master node responsible for controlling the cluster state. These configurations are usually held in the opensearch.yml config file, environment variables on the node, or within the cluster state.
Node Roles – In small clusters it is common for all nodes to fill all roles; all nodes can store data, become master nodes or process ingestion pipelines. However as the cluster grows, it is common to allocate specific roles to specific nodes in order to simplify configuration and to make operation more efficient. In particular, it is common to define a limited number of dedicated master nodes.
Replication – Data may be replicated across a number of data nodes. This means that if one node goes down, data is not lost. It also means that a search request can be dealt with by more than one node.
Common problems
Many OpenSearch problems are caused by operations which place an excessive burden on the cluster because they require an excessive amount of information to be held and transmitted between the nodes as part of the cluster state. For example:
- Shards too small
- Too many fields (field explosion)
Problems may also be caused by inadequate configurations causing situations where the OpenSearch cluster is unable to safely elect a Master node. These type problems include:
- Master node not discovered
- Split brain problem
Backups
Because OpenSearch is a clustered technology, it is not sufficient to have backups of each node’s data directory. This is because the backups will have been made at different times and so there may not be complete coherency between them. As such, the only way to backup an OpenSearch cluster is through the use of snapshots, which contain the full picture of an index at any one time.
Cluster resilience
When designing an OpenSearch cluster, it is important to think about cluster resilience. In particular – what happens when a single node goes down? And for larger clusters where several nodes may share common services such as a network or power supply – what happens if that network or power supply goes down? This is where it is useful to ensure that the master eligible nodes are spread across availability zones, and to use shard allocation awareness to ensure that shards are spread across different racks or availability zones in your data center.
Log Context
Log “low disk watermark [{}] no longer exceeded on {}” classname is DiskThresholdMonitor.java.
We extracted the following from OpenSearch source code for those seeking an in-depth context :
reroute = true;
explanation = "one or more nodes has gone under the high or low watermark";
nodesOverLowThreshold.remove(node);
nodesOverHighThreshold.remove(node);
logger.info("low disk watermark [{}] no longer exceeded on {}";
diskThresholdSettings.describeLowThreshold(); usage);
} else {
logger.debug("{} has gone below a disk threshold; but an automatic reroute has occurred " +
"in the last [{}]; skipping reroute";
