In all three places where _secondaryThrottle is mentioned
http://docs.mongodb.org/manual/tutorial/manage-chunks-in-sharded-cluster/
http://docs.mongodb.org/manual/tutorial/configure-sharded-cluster-balancer/#require-replication-before-chunk-migration-secondary-throttle
http://docs.mongodb.org/manual/reference/command/moveChunk/#movechunk

the following is unclear or needs to be fixed (for 2.4):

In 2.4 the default setting of _secondaryThrottle is 'true' (was false before).

When it's set to true, the following is what those three pages say:

"When set to true, MongoDB ensures replication to secondaries before allowing new chunk migrations."

• this is misleading - MongoDB always ensures replication to majority before even entering critical section of migration of each chunk. This option will slow down copying and deleting by ensuring replication (to one secondary) of every single document during copy/delete. The key is this option add continuous checking of replication instead of "one time" basically.

"When enabled, secondary throttle puts a

write concern on deletes and on copies, which means the balancer waits for those operations to replicate to at least one secondary before migrating chunks.When enabled, secondary throttle puts a

write concern on deletes and on copies, which means the balancer waits for those operations to replicate to at least one secondary before migrating chunks."

This is exactly correct, except for the last clause - should be "before continuing with the next document" - this is happening in the midst of chunk migration.

"_secondaryThrottle (boolean) – Optional. Set to false by default. If set to true, the balancer waits for replication to secondaries while copying and deleting data during migrations."

This is 100% correct (for 2.2) I just want to make sure it got changed to "Set to true by default" (for 2.4).