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Elasticsearch Reference [7.5] » Modules » Shard allocation and cluster-level routing » Disk-based shard allocation
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Disk-based shard allocationedit

Elasticsearch considers 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 that Elasticsearch will not allocate shards to nodes that have more than 85% disk used. It can also be set to an absolute byte value (like 500mb) to prevent Elasticsearch from allocating shards if less than the specified amount of space is available. This setting has no effect on the primary shards of newly-created indices but will prevent their replicas from being allocated.
cluster.routing.allocation.disk.watermark.high
Controls the high watermark. It defaults to 90%, meaning that Elasticsearch will attempt to relocate shards away from a node whose disk usage is above 90%. It can also be set to an absolute byte value (similarly to the low watermark) to relocate shards away from a node if it has less than the specified amount of free space. This setting affects the allocation of all shards, whether previously allocated or not.
cluster.routing.allocation.disk.watermark.flood_stage

Controls the flood stage watermark. It defaults to 95%, meaning that Elasticsearch enforces a read-only index block (index.blocks.read_only_allow_delete) on every index that has one or more shards allocated on the node that has at least one disk exceeding the flood stage. This is a last resort to prevent nodes from running out of disk space. The index block is automatically released once the disk utilization falls below the high watermark.

You can not mix the usage of percentage values and byte values within these settings. Either all are set to percentage values, or all are set to byte values. This is so that we can we validate that the settings are internally consistent (that is, the low disk threshold is not more than the high disk threshold, and the high disk threshold is not more than the flood stage threshold).

An example of resetting the read-only index block on the twitter index:

PUT /twitter/_settings
{
  "index.blocks.read_only_allow_delete": null
}
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
[7.5.0] Deprecated in 7.5.0. Future versions will always account for 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.

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.

An example of updating the low watermark to at least 100 gigabytes free, a high watermark of at least 50 gigabytes free, and a flood stage watermark of 10 gigabytes free, and updating the information about the cluster every minute:

PUT _cluster/settings
{
  "transient": {
    "cluster.routing.allocation.disk.watermark.low": "100gb",
    "cluster.routing.allocation.disk.watermark.high": "50gb",
    "cluster.routing.allocation.disk.watermark.flood_stage": "10gb",
    "cluster.info.update.interval": "1m"
  }
}
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