CODEWITHSUNDEEP
cassandra
piotrwest
piotrwest

Reputation: 2166

What does Cassandra nodetool repair exactly do?

From http://docs.datastax.com/en/cassandra/2.0/cassandra/operations/ops_repair_nodes_c.html I know that

The nodetool repair command repairs inconsistencies across all of the replicas for a given range of data.

Question aside: it's compaction which evicts tombstones, right? So the requirement for running nodetool repair more frequently than gc_grace seconds is only to ensure that all data is spread to appropriate replicas? Shouldn't be that the usual scenario?

Upvotes: 3

Views: 5540

Answers (1)

Jim Meyer
Jim Meyer

Reputation: 9475

The data can become inconsistent whenever a write to a replica is not completed for whatever reason. This can happen if a node is down, if the node is up but the network connection is down, if a queue fills up and the write is dropped, disk failure, etc.

When inconsistent data is detected by comparing the merkle trees, the bad sections of data are repaired by streaming them from the nodes with the newer data. Streaming is a basic mechanism in Cassandra and is also used for bootstrapping empty nodes into the cluster.

The reason you need to run repair within gc grace seconds is so that tombstones will be sync'd to all nodes. If a node is missing a tombstone, then it won't drop that data during compaction. The nodes with the tombstone will drop the data during compaction, and then when they later run repair, the deleted data can be resurrected from the node that was missing the tombstone.

Upvotes: 5

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