Disk-based Shard Allocation

Elasticsearch factors in the available disk space on a node before deciding whether to allocate new shards to that node or to actively relocate shards away from that node.

Below are the settings that can be configured in the elasticsearch.yml config file or updated dynamically on a live cluster with the cluster-update-settings API:

cluster.routing.allocation.disk.threshold_enabled
Defaults to true. Set to false to disable the disk allocation decider.
cluster.routing.allocation.disk.watermark.low
Controls the low watermark for disk usage. It defaults to 85%, meaning ES will not allocate new shards to nodes once they have more than 85% disk used. It can also be set to an absolute byte value (like 500mb) to prevent ES from allocating shards if less than the configured amount of space is available.
cluster.routing.allocation.disk.watermark.high
Controls the high watermark. It defaults to 90%, meaning ES will attempt to relocate shards to another node if the node disk usage rises above 90%. It can also be set to an absolute byte value (similar to the low watermark) to relocate shards once less than the configured amount of space is available on the node.

Percentage values refer to used disk space, while byte values refer to free disk space. This can be confusing, since it flips the meaning of high and low. For example, it makes sense to set the low watermark to 10gb and the high watermark to 5gb, but not the other way around.

cluster.info.update.interval
How often Elasticsearch should check on disk usage for each node in the cluster. Defaults to 30s.
cluster.routing.allocation.disk.include_relocations
Defaults to true, which means that Elasticsearch will take into account shards that are currently being relocated to the target node when computing a node’s disk usage. Taking relocating shards' sizes into account may, however, mean that the disk usage for a node is incorrectly estimated on the high side, since the relocation could be 90% complete and a recently retrieved disk usage would include the total size of the relocating shard as well as the space already used by the running relocation.

An example of updating the low watermark to no more than 80% of the disk size, a high watermark of at least 50 gigabytes free, and updating the information about the cluster every minute:

PUT /_cluster/settings
{
  "transient": {
    "cluster.routing.allocation.disk.watermark.low": "80%",
    "cluster.routing.allocation.disk.watermark.high": "50gb",
    "cluster.info.update.interval": "1m"
  }
}

Prior to 2.0.0, when using multiple data paths, the disk threshold decider only factored in the usage across all data paths (if you had two data paths, one with 50b out of 100b free (50% used) and another with 40b out of 50b free (80% used) it would see the node’s disk usage as 90b out of 150b). In 2.0.0, the minimum and maximum disk usages are tracked separately.