[[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.

`cluster.routing.allocation.disk.watermark.flood_stage`::
+
--
Controls the flood stage watermark. It defaults to 95%, meaning ES 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 must be released manually
once there is enough disk space available to allow indexing operations to
continue.

NOTE: 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:

[source,js]
--------------------------------------------------
PUT /twitter/_settings
{
  "index.blocks.read_only_allow_delete": null
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]
--

`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.


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.

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:

[source,js]
--------------------------------------------------
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"
  }
}
--------------------------------------------------
// CONSOLE