// Module included in the following assemblies:
//
// * observability/logging/cluster-logging-deploying.adoc

:_mod-docs-content-type: CONCEPT
[id="logging-es-storage-considerations_{context}"]
= Storage considerations for Elasticsearch

A persistent volume is required for each Elasticsearch deployment configuration. On {product-title} this is achieved using persistent volume claims (PVCs).

[NOTE]
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If you use a local volume for persistent storage, do not use a raw block volume, which is described with `volumeMode: block` in the `LocalVolume` object. Elasticsearch cannot use raw block volumes.
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The OpenShift Elasticsearch Operator names the PVCs using the Elasticsearch resource name.

Fluentd ships any logs from *systemd journal* and **/var/log/containers/*.log** to Elasticsearch.

Elasticsearch requires sufficient memory to perform large merge operations. If it does not have enough memory, it becomes unresponsive. To avoid this problem, evaluate how much application log data you need, and allocate approximately double that amount of free storage capacity.

By default, when storage capacity is 85% full, Elasticsearch stops allocating new data to the node. At 90%, Elasticsearch attempts to relocate existing shards from that node to other nodes if possible. But if no nodes have a free capacity below 85%, Elasticsearch effectively rejects creating new indices and becomes RED.

[NOTE]
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These low and high watermark values are Elasticsearch defaults in the current release. You can modify these default values. Although the alerts use the same default values, you cannot change these values in the alerts.
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