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This guide will help you check for common problems that cause the log ” failed to prepare shard for recovery ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: recovery, indices and shard.
Overview
In Elasticsearch, recovery refers to the process of recovering an index or shard when something goes wrong. There are many ways to recover an index or shard, such as by re-indexing the data from a backup / failover cluster to the current one, or by restoring from an Elasticsearch snapshot. Alternatively, Elasticsearch performs recoveries automatically, such as when a node restarts or disconnects and connects again. There is an API to check the updated status of index / shard recoveries.
GET /<index>/_recoveryGET /_recovery
In summary, recovery can happen in the following scenarios:
- Node startup or failure (local store recovery)
- Replication of primary shards to replica shards
- Relocation of a shard to a different node in the same cluster
- Restoring a snapshot
Examples
Getting recovery information about several indices:
GET my_index1 GET my_index2/_recovery
Notes and good things to know
- When a node is disconnected from the cluster, all of its shards go to an unassigned state. After a certain amount of time, the shards will be allocated somewhere else on other nodes. This setting determines the number of concurrent shards per node that will be recovered.
PUT _cluster/settings{"transient":{"cluster.routing.allocation.node_concurrent_recoveries":3}}- You can also control when to start recovery after a node disconnects. This is useful if the node just restarts, for example, because you may not want to initiate any recovery for such transient events.
PUT _all/_settings{"settings":{"index.unassigned.node_left.delayed_timeout":"6m"}}- Elasticsearch limits the speed that is allocated to recovery in order to avoid overloading the cluster. This setting can be updated to make the recovery faster or slower, depending on your requirements.
PUT _cluster/settings{"transient":{"indices.recovery.max_bytes_per_sec":"100mb"}}Log Context
Log “failed to prepare shard for recovery”classname is PeerRecoveryTargetService.java We extracted the following from Elasticsearch source code for those seeking an in-depth context :
actionName = PeerRecoverySourceService.Actions.START_RECOVERY;
} catch (final Exception e) {
// this will be logged as warning later on...
logger.trace("unexpected error while preparing shard for peer recovery; failing recovery"; e);
onGoingRecoveries.failRecovery(recoveryId;
new RecoveryFailedException(recoveryTarget.state(); "failed to prepare shard for recovery"; e); true);
return;
}
logger.trace("{} starting recovery from {}"; startRequest.shardId(); startRequest.sourceNode());
} else {
startRequest = preExistingRequest;
Find & fix Elasticsearch problems
