[[disk-allocator]]
=== 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,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.

NOTE: 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:

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

NOTE: 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.
