[[disk-allocator]]
=== Disk-based shard allocation

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,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 or, specifically, any shards that have
    never previously been 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 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