Overview
OpenSearch uses several parameters to enable it to manage hard disk storage across the cluster.
What it’s used for
- OpenSearch will actively try to relocate shards away from nodes which exceed the disk watermark high threshold.
- OpenSearch will NOT locate new shards or relocate shards on to nodes which exceed the disk watermark low threshold.
- OpenSearch will prevent all writes to an index which has any shard on a node that exceeds the disk.watermark.flood_stage threshold.
- The info update interval is the time it will take OpenSearch to re-check the disk usage.
Examples
PUT _cluster/settings
{
"transient": {
"cluster.routing.allocation.disk.watermark.low": "85%",
"cluster.routing.allocation.disk.watermark.high": "90%",
"cluster.routing.allocation.disk.watermark.flood_stage": "95%",
"cluster.info.update.interval": "1m"
}
}Notes and good things to know
- You can use absolute values (100gb) or percentages (90%), but you cannot mix the two on the same cluster.
- In general, it is recommended to use percentages, since this will work in case the disks are resized.
- You can put the cluster settings on the opensearch.yml of each node, but it is recommended to use the PUT _cluster/settings API because it is easier to manage, and ensures that the settings are coherent across the cluster.
- OpenSearch comes with sensible defaults for these settings, so think twice before modifying them. If you find you are spending a lot of time fine-tuning these settings, then it is probably time to invest in new disk space.
- In the event of the flood_stage threshold being exceeded, once you delete data, OpenSearch should detect automatically that the block can be released (bearing in mind the update interval which could be, for instance, a minute). However if you want to accelerate this process, you can unblock an index manually, with the following call:
PUT /my_index/_settings
{
"index.blocks.read_only_allow_delete": null
}Common problems
Inappropriate cluster settings (if the disk watermark.low is too low) can make it impossible for OpenSearch to allocate shards on the cluster. In particular, bear in mind that these parameters work in combination with other cluster settings (for example shard allocation awareness) which cause further restraints on how OpenSearch can allocate shards.
