Here’s how to resolve the most common causes of unassigned shards reported by the cluster allocation explain API.
- Disk is full
- A node containing data has moved to a different host
- Unable to assign shards based on the allocation rule
- The number of eligible data nodes is less than the number of replicas
- A snapshot issue prevents searchable snapshot indices from being allocated
- Maximum retry times exceeded
- Max shard per node reached the limit
If the disk usage exceeded the threshold, you may get one or more of the following messages:
the node is above the high watermark cluster setting [cluster.routing.allocation.disk.watermark.high=90%], using more disk space than the maximum allowed [90.0%], actual free: [9.273781776428223%]
unable to force allocate shard to [%s] during replacement, as allocating to this node would cause disk usage to exceed 100%% ([%s] bytes above available disk space)
the node is above the low watermark cluster setting [cluster.routing.allocation.disk.watermark.low=85%], using more disk space than the maximum allowed [85.0%], actual free: [14.119771122932434%]
after allocating [[restored-xxx], node[null], [P], recovery_source[snapshot recovery [Om66xSJqTw2raoNyKxsNWg] from xxx/W5Yea4QuR2yyZ4iM44fumg], s[UNASSIGNED], unassigned_info[[reason=NEW_INDEX_RESTORED], at[2022-03-02T10:56:58.210Z], delayed=false, details[restore_source[xxx]], allocation_status[fetching_shard_data]]] node [GTXrECDRRmGkkAnB48hPqw] would have more than the allowed 10% free disk threshold (8.7% free), preventing allocation
Check also Cluster-level shard allocation and routing settings for more information.
Increase the disk size.
When the disk usage exceeds the flood-stage watermark and the deployment is already unhealthy, you must free up disk space by deleting unused indices before you can increase the disk size. Attempting to make configuration changes while the cluster is unhealthy will fail. For more information, refer to How can I customize the components of my deployment?.
This helps you manage your deployments more easily by automatically adjusting capacity. You might need to delete unused indices to free up disk space before you can enable the autoscaling feature.
- Configure ILM policies to migrate older data to lower-cost data tiers and use searchable snapshots to provide redundancy for read-only data. You must restore the cluster to a healthy state before you can add data tiers. For more information, see data tiers and ILM policy.
During the routine system maintenance performed by Elastic, it might happen that a node moves to a different host. If the indices are not configured with replica shards, the shard data on the Elasticsearch node that is moved will be lost, and you might get one or more of these messages:
cannot allocate because a previous copy of the primary shard existed but can no longer be found on the nodes in the cluster
Configure an highly available cluster to keep your service running. Also, consider taking the following actions to bring your deployment back to health and recover your data from the snapshot.
For more information, check also Designing for resilience.
node does not match index setting [index.routing.allocation.include] filters [node_type:\"cold\"]
index has a preference for tiers [data_cold] and node does not meet the required [data_cold] tier
index has a preference for tiers [data_cold,data_warm,data_hot] and node does not meet the required [data_cold] tier
index has a preference for tiers [data_warm,data_hot] and node does not meet the required [data_warm] tier
this node's data roles are exactly [data_frozen] so it may only hold shards from frozen searchable snapshots, but this index is not a frozen searchable snapshot
- Make sure nodes are available in each data tier and have sufficient disk space.
- Check the index settings and ensure shards can be allocated to the expected data tier.
- Check the ILM policy and check for issues with the allocate action.
- Inspect the index templates and check for issues with the index settings.
Unassigned replica shards are often caused by there being fewer eligible data nodes than the configured number_of_replicas.
Some snapshots operations might be impacted, as shown in the following example:
failed shard on node [Yc_Jbf73QVSVYSqZT8HPlA]: failed recovery, failure RecoveryFailedException[[restored-my_index-2021.32]: … SnapshotMissingException[[found-snapshots:2021.08.25-my_index-2021.32-default_policy-_j2k8it9qnehe1t-2k0u6a/iOAoyjWLTyytKkW3_wF1jw] is missing]; nested: NoSuchFileException[Blob object [snapshots/52bc3ae2030a4df8ab10559d1720a13c/indices/WRlkKDuPSLW__M56E8qbfA/1/snap-iOAoyjWLTyytKkW3_wF1jw.dat] not found: The specified key does not exist. (Service: Amazon S3; Status Code: 404; Error Code: NoSuchKey; Request ID: 4AMTM1XFMTV5F00V; S3 Extended Request ID:
Upgrade to Elasticsearch version 7.17.0 or later, which resolves bugs that affected snapshot operations in earlier versions. Check Upgrade versions for more details.
If you can’t upgrade, you can recreate the snapshot repository as a workaround.
The bugs also affect searchable snapshots. If you still have data in the cluster but cannot restore from the searchable snapshot, you can try reindexing and recreating the searchable snapshot:
- Reindex all the affected indices to new regular indices
- Remove the affected frozen indices
- Take the snapshot and mount the indices again
cluster.max_shards_per_node limits the total number of primary and replica shards for the cluster. If your cluster has a number of shards beyond this limit, you might get the following message:
Validation Failed: 1: this action would add  shards, but this cluster currently has / maximum normal shards open
Delete unnecessary indices, add more data nodes, and avoid oversharding as too many shards can overwhelm your cluster. If you cannot take these actions, and you’re confident your changes won’t destabilize the cluster, you can temporarily increase the limit using the cluster update settings API and retry the action. For more details, check Troubleshoot shard-related errors.
The cluster will attempt to allocate a shard a few times, before giving up and leaving the shard unallocated. On Elasticsearch Service,
index.allocation.max_retries defaults to 5. If allocation fails after the maximum number of retries, you might get the following message:
shard has exceeded the maximum number of retries [%d] on failed allocation attempts - manually call [/_cluster/reroute?retry_failed=true] to retry, [%s]