Watermark errors

When a data node is critically low on disk space and has reached the flood-stage disk usage watermark, the following error is logged: Error: disk usage exceeded flood-stage watermark, index has read-only-allow-delete block.

To prevent a full disk, when a node reaches this watermark, Elasticsearch blocks writes to any index with a shard on the node. If the block affects related system indices, Kibana and other Elastic Stack features may become unavailable. For example, this could induce Kibana's Kibana Server is not Ready yet error message.

Elasticsearch will automatically remove the write block when the affected node’s disk usage falls below the high disk watermark. To achieve this, Elasticsearch attempts to rebalance some of the affected node’s shards to other nodes in the same data tier.

Admonition

If you’re using Elastic Cloud Hosted, then you can use AutoOps to monitor your cluster. AutoOps significantly simplifies cluster management with performance recommendations, resource utilization visibility, real-time issue detection and resolution paths. For more information, refer to Monitor with AutoOps.

Monitor rebalancing ¶

To verify that shards are moving off the affected node until it falls below high watermark., use the cat shards API and cat recovery API:

			GET _cat/shards?v=true
			
GET _cat/recovery?v=true&active_only=true

If shards remain on the node keeping it about high watermark, use the cluster allocation explanation API to get an explanation for their allocation status.

			GET _cluster/allocation/explain
			{
  "index": "my-index",
  "shard": 0,
  "primary": false
}

Temporary Relief ¶

To immediately restore write operations, you can temporarily increase disk watermarks and remove the write block.

			PUT _cluster/settings
			{
  "persistent": {
    "cluster.routing.allocation.disk.watermark.low": "90%",
    "cluster.routing.allocation.disk.watermark.low.max_headroom": "100GB",
    "cluster.routing.allocation.disk.watermark.high": "95%",
    "cluster.routing.allocation.disk.watermark.high.max_headroom": "20GB",
    "cluster.routing.allocation.disk.watermark.flood_stage": "97%",
    "cluster.routing.allocation.disk.watermark.flood_stage.max_headroom": "5GB",
    "cluster.routing.allocation.disk.watermark.flood_stage.frozen": "97%",
    "cluster.routing.allocation.disk.watermark.flood_stage.frozen.max_headroom": "5GB"
  }
}

PUT */_settings?expand_wildcards=all
{
  "index.blocks.read_only_allow_delete": null
}

When a long-term solution is in place, to reset or reconfigure the disk watermarks:

			PUT _cluster/settings
			{
  "persistent": {
    "cluster.routing.allocation.disk.watermark.low": null,
    "cluster.routing.allocation.disk.watermark.low.max_headroom": null,
    "cluster.routing.allocation.disk.watermark.high": null,
    "cluster.routing.allocation.disk.watermark.high.max_headroom": null,
    "cluster.routing.allocation.disk.watermark.flood_stage": null,
    "cluster.routing.allocation.disk.watermark.flood_stage.max_headroom": null,
    "cluster.routing.allocation.disk.watermark.flood_stage.frozen": null,
    "cluster.routing.allocation.disk.watermark.flood_stage.frozen.max_headroom": null
  }
}

Resolve ¶

To resolve watermark errors permanently, perform one of the following actions:

  • Horizontally scale nodes of the affected data tiers.
  • Vertically scale existing nodes to increase disk space.
  • Delete indices using the delete index API, either permanently if the index isn’t needed, or temporarily to later restore.
  • update related ILM policy to push indices through to later data tiers

Tip

On Elasticsearch Service and Elastic Cloud Enterprise, indices may need to be temporarily deleted via its Elasticsearch API Console to later snapshot restore in order to resolve cluster health status:red which will block attempted changes. If you experience issues with this resolution flow on Elasticsearch Service, kindly reach out to Elastic Support for assistance.