ElasticSearch index recovery
I am new to ElasticSearch and I am confused about the meaning of index recovery. What are the operations index recovery performs? Does it mean recovering the data inside the index or allocating unassigned shards?
Index recovery means loading shards from disk and making it usable for your query operations. This can happen if you start a node, make new replicas, add or remove new node to your cluster or if some node has crashed and restarting. There can be multiple operations involved in the process. If a shard is coming up, it will ask all other shards what data they have and try to do an integrity check. If a new node has been added and there is no shared disk, then there will be data movements. If a new primary shard is to be selected, then primary should be the one holding most of the data at that time, so nodes need to to be in sync. To handle all these cases, there must be dozens of other tasks being done in recovery process.
According to ElasticSearch Reference: A recovery event occurs anytime an index shard moves to a different node in the cluster. This can happen during a snapshot recovery, a change in replication level, node failure, or on node startup. This last type is called a local store recovery and is the normal way for shards to be loaded from disk when a node starts up.
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