[[disk-based-shard-allocation]]
==== Disk-based shard allocation settings
[[disk-based-shard-allocation-description]]
// tag::disk-based-shard-allocation-description-tag[]
{es} 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.

You can use the following settings to control disk-based allocation:

[[cluster-routing-disk-threshold]]
// tag::cluster-routing-disk-threshold-tag[]
`cluster.routing.allocation.disk.threshold_enabled` {ess-icon}::
(<<dynamic-cluster-setting,Dynamic>>)
Defaults to `true`.  Set to `false` to disable the disk allocation decider.
// end::cluster-routing-disk-threshold-tag[]

[[cluster-routing-watermark-low]]
// tag::cluster-routing-watermark-low-tag[]
`cluster.routing.allocation.disk.watermark.low` {ess-icon}::
(<<dynamic-cluster-setting,Dynamic>>)
Controls the low watermark for disk usage. It defaults to `85%`, meaning that {es} 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 {es} 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.
// end::cluster-routing-watermark-low-tag[]

[[cluster-routing-watermark-high]]
// tag::cluster-routing-watermark-high-tag[]
`cluster.routing.allocation.disk.watermark.high` {ess-icon}::
(<<dynamic-cluster-setting,Dynamic>>)
Controls the high watermark. It defaults to `90%`, meaning that {es} 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.
// end::cluster-routing-watermark-high-tag[]

`cluster.routing.allocation.disk.watermark.enable_for_single_data_node`::
    (<<static-cluster-setting,Static>>)
    For a single data node, the default is to disregard disk watermarks when
    making an allocation decision. This is deprecated behavior and will be
    changed in 8.0. This setting can be set to `true` to enable the
    disk watermarks for a single data node cluster (will become default in 8.0).

[[cluster-routing-flood-stage]]
// tag::cluster-routing-flood-stage-tag[]
`cluster.routing.allocation.disk.watermark.flood_stage` {ess-icon}::
+
--
(<<dynamic-cluster-setting,Dynamic>>)
Controls the flood stage watermark, which defaults to 95%. {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, and that has at least one disk exceeding the flood stage. This setting is a last resort to prevent nodes from running out of disk space. The index block is automatically released when the disk utilization falls below the high watermark.

NOTE: You cannot mix the usage of percentage values and byte values within
these settings. Either all values are set to percentage values, or all are set to byte values. This enforcement is so that {es} can validate that the settings are internally consistent, ensuring that the low disk threshold is less than the high disk threshold, and the high disk threshold is less than the flood stage threshold.

An example of resetting the read-only index block on the `my-index-000001` index:

[source,console]
--------------------------------------------------
PUT /my-index-000001/_settings
{
  "index.blocks.read_only_allow_delete": null
}
--------------------------------------------------
// TEST[setup:my_index]
--
// end::cluster-routing-flood-stage-tag[]

`cluster.info.update.interval`::
    (<<dynamic-cluster-setting,Dynamic>>)
    How often {es} should check on disk usage for each node in the
    cluster. Defaults to `30s`.

`cluster.routing.allocation.disk.include_relocations`::

    deprecated:[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.


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,console]
--------------------------------------------------
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"
  }
}
--------------------------------------------------
