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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 the index.recovery.initial_shards setting is satisfied.

cluster.routing.allocation.node_concurrent_recoveries
How many concurrent shard recoveries are allowed to happen on a node. Defaults to 2.
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.
indices.recovery.concurrent_streams
The number of network streams to open per node to recover a shard from a peer shard. Defaults to 3.
indices.recovery.concurrent_small_file_streams
The number of streams to open per node for small files (under 5mb) to recover a shard from a peer shard. Defaults to 2.

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 - (default) Always allow rebalancing.
  • indices_primaries_active - Only when all primaries in the cluster are allocated.
  • indices_all_active - 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.

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 action can bring the weights of each node closer together by more then the balance.threshold.

cluster.routing.allocation.balance.shard
Defines the weight factor for 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 a factor to 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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