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Cluster Level Shard Allocation

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Cluster Level Shard Allocation

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Shard allocation is the process of allocating shards to nodes. This can happen during initial recovery, replica allocation, rebalancing, or when nodes are added or removed.

Shard Allocation Settings

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The following dynamic settings may be used to control shard allocation and recovery:

cluster.routing.allocation.enable

Enable or disable allocation for specific kinds of shards:

all - (default) Allows shard allocation for all kinds of shards.
primaries - Allows shard allocation only for primary shards.
new_primaries - Allows shard allocation only for primary shards for new indices.
none - No shard allocations of any kind are allowed for any indices.

This setting does not affect the recovery of local primary shards when restarting a node. A restarted node that has a copy of an unassigned primary shard will recover that primary immediately, assuming that its allocation id matches one of the active allocation ids in the cluster state.

cluster.routing.allocation.node_concurrent_incoming_recoveries

How many concurrent incoming shard recoveries are allowed to happen on a node. Incoming recoveries are the recoveries where the target shard (most likely the replica unless a shard is relocating) is allocated on the node. Defaults to 2.

cluster.routing.allocation.node_concurrent_outgoing_recoveries

How many concurrent outgoing shard recoveries are allowed to happen on a node. Outgoing recoveries are the recoveries where the source shard (most likely the primary unless a shard is relocating) is allocated on the node. Defaults to 2.

cluster.routing.allocation.node_concurrent_recoveries

A shortcut to set both cluster.routing.allocation.node_concurrent_incoming_recoveries and cluster.routing.allocation.node_concurrent_outgoing_recoveries.

cluster.routing.allocation.node_initial_primaries_recoveries

While the recovery of replicas happens over the network, the recovery of an unassigned primary after node restart uses data from the local disk. These should be fast so more initial primary recoveries can happen in parallel on the same node. Defaults to 4.

cluster.routing.allocation.same_shard.host

Allows to perform a check to prevent allocation of multiple instances of the same shard on a single host, based on host name and host address. Defaults to false, meaning that no check is performed by default. This setting only applies if multiple nodes are started on the same machine.

Shard Rebalancing Settings

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The following dynamic settings may be used to control the rebalancing of shards across the cluster:

cluster.routing.rebalance.enable

Enable or disable rebalancing for specific kinds of shards:

all - (default) Allows shard balancing for all kinds of shards.
primaries - Allows shard balancing only for primary shards.
replicas - Allows shard balancing only for replica shards.
none - No shard balancing of any kind are allowed for any indices.

cluster.routing.allocation.allow_rebalance

Specify when shard rebalancing is allowed:

always - Always allow rebalancing.
indices_primaries_active - Only when all primaries in the cluster are allocated.
indices_all_active - (default) Only when all shards (primaries and replicas) in the cluster are allocated.

cluster.routing.allocation.cluster_concurrent_rebalance

Allow to control how many concurrent shard rebalances are allowed cluster wide. Defaults to 2. Note that this setting only controls the number of concurrent shard relocations due to imbalances in the cluster. This setting does not limit shard relocations due to allocation filtering or forced awareness.

Shard Balancing Heuristics

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The following settings are used together to determine where to place each shard. The cluster is balanced when no allowed rebalancing operation can bring the weight of any node closer to the weight of any other node by more than the balance.threshold.

cluster.routing.allocation.balance.shard: Defines the weight factor for the total number of shards allocated on a node (float). Defaults to 0.45f. Raising this raises the tendency to equalize the number of shards across all nodes in the cluster.
cluster.routing.allocation.balance.index: Defines the weight factor for the number of shards per index allocated on a specific node (float). Defaults to 0.55f. Raising this raises the tendency to equalize the number of shards per index across all nodes in the cluster.
cluster.routing.allocation.balance.threshold: Minimal optimization value of operations that should be performed (non negative float). Defaults to 1.0f. Raising this will cause the cluster to be less aggressive about optimizing the shard balance.

Regardless of the result of the balancing algorithm, rebalancing might not be allowed due to forced awareness or allocation filtering.

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