Elasticsearch is a memory-intensive application. The default cluster logging installation deploys 16G of memory for both memory requests and memory limits. The initial set of OKD nodes might not be large enough to support the Elasticsearch cluster. You must add additional nodes to the OKD cluster to run with the recommended or higher memory. Each Elasticsearch node can operate with a lower memory setting, though this is not recommended for production deployments.
A persistent volume is required for each Elasticsearch deployment configuration. On OKD this is achieved using Persistent Volume Claims.
The Elasticsearch Operator names the PVCs using the Elasticsearch resource name.
Fluentd ships any logs from systemd journal and /var/log/containers/*.log to Elasticsearch.
Therefore, consider how much data you need in advance and that you are aggregating application log data. Some Elasticsearch users have found that it is necessary to keep absolute storage consumption around 50% and below 70% at all times. This helps to avoid Elasticsearch becoming unresponsive during large merge operations.
By default, at 85% 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 free capacity below 85%, Elasticsearch effectively rejects creating new indices and becomes RED.
These low and high watermark values are Elasticsearch defaults in the current release. You can modify these values, but you also must apply any modifications to the alerts also. The alerts are based on these defaults.