Cassandra nodetool repair parallelism: "parallel" versus "dc_parallel"

Question

If I run nodetool repair --full I see settings like this:

repairing keyspace some_keyspace with repair options
  (parallelism: parallel, primary range: false, incremental: false,
   job threads: 1, ColumnFamilies: [], dataCenters: [], hosts: [], # of ranges: 735)

But if I run nodetool repair --full --dc-parallel I see this:

repairing keyspace some_other_keyspace with repair options
  (parallelism: dc_parallel, primary range: false, incremental: false,
   job threads: 1, ColumnFamilies: [], dataCenters: [], hosts: [], # of ranges: 1992)

Note the parallelism setting. Since Cassandra 2.2, "parallel" has become the default. I was wondering what the difference is between "parallel" and "dc_parallel". The documentation implies that dc_parallel is faster because it operates in all DCs at once, but in my brief testing it seems to be the opposite; perhaps it is due to my replication settings – most keyspaces are set to use NetworkTopologyStrategy and place an equal number of replicas in each of my DCs – but I would like to be more sure than just a guess. "parallel" is not well documented.

For what it's worth, I am running Cassandra 3 (DSE 5.0.3.)

Answer 1

The problem here is the poor choice of name for the --dc-parallel option, IMO.

Currently there are 3 "parallelism degrees" that is possible to specify for a repair operation: sequential, parallel and datacenter-aware (dc-parallel).

• Parallel repair is the oldest option and it simply allows all nodes to run repair tasks at once. This can be pretty CPU-intensive.
• Sequential repair was introduced in order to reduce the CPU impact of repair on the whole cluster. It is implemented by taking snapshots on all affected nodes then performing Merkle tree calculations using snapshots, one node at a time. It puts less pressure on the CPU at a cost of longer run time and additional space taken by the snapshots.
• Finally, "dc-parallel" repair is actually sequential repair within every DC, while all DCs are allowed to run these repairs at the same time. So with this option only one node per DC is actively working on repair at a time. This is not different from sequential if you only have a single DC.