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What is the difference between:
a) nodetool rebuild
b) nodetool repair [-pr]
In other words, what exactly do the respective commands do?
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Repair synchronizes the data between nodes by comparing their respective datasets for their common token ranges, and streaming the differences for any out of sync sections between the nodes. It compares the data with merkle trees, which are a hierarchy of hashes.
Since Cassandra has a distributed architecture, it is necessary to run repairs to keep the copies of the data consistent between replicas (Cassandra nodes). Repairs need to be run at least once every gc_grace_seconds (configured per table).
Repairs one or more tables. The repair command repairs one or more nodes in a cluster, and provides options for restricting repair to a set of nodes, see Repairing nodes. Performing an anti-entropy node repair on a regular basis is important, especially in an environment that deletes data frequently.
A full repair of all SSTables on a node takes a lot of time and is resource-intensive. You can manage repairs with less service disruption using incremental repair. Incremental repair consumes less time and resources because it skips SSTables that are already marked as repaired.
nodetool rebuild: is similar to the bootstrapping process (when you add a new node to the cluster) but for a datacenter. The process here is mainly a streaming from the already live nodes to the new nodes (the new ones are empty). So after defining the key ranges for the nodes which is very fast, the rest can be seen as a copy operation.
nodetool repair -pr: is not a copy operation, the node being repaired is not empty, it already contains data but if the replication factor is greater than 1 that data needs to be compared to the data on the rest of the replicas and if there is a difference it will be corrected. The process involves a lot of streaming but it is not data streaming: the node being repaired requests a merkle tree (basically a tree of hashes) in order to verify if the information both nodes have is the same or not, if not it requests a full stream of the section of the data that has any difference (so all the replicas have the same data). Streaming this hashes if faster than streaming the whole data before verification, this works under the assumption that most data will be the same on both nodes except for some differences here and there. This process also removes tombstones created when deleting from the database, defining like a new "checkpoint" after which new tombstones will be created upon deletion of data, but the old ones will not be used anymore.
Hope it helps!
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