> That's finally a precise statement! :) I was wondering what " to at least 1 > replica's commit log" is supposed to actually mean: > http://wiki.apache.org/cassandra/API
The main idea is that it has been "officially delivered" to one replicate. If Cassandra only did batch-wise commit such that a write was never ACK:ed until it was durable, it would mean that it had been durably written to 1 replica set. I suspect the phrasing is to get around the fact that it is not actually durably written if nodes are configured to use periodic sync mode. > Does quorum mean that data is replicated to q nodes or to at least q nodes? That it is replicated to at least a quorom of nodes before the write is considered successful. This does not prevent further propagation to all nodes; data always gets replicated according to replication factor. Consistency levels only affect the consistency requirements of the particular request. > I just added another blank machine to my cluster. Nothing happened as > expected (stopped writing to the cluster) but after I ran nodetool repair it > held more data than all other nodes. So it copied data from the other nodes > to this one? I assumed that data is replicated to q nodes not to all, is > quorum 'only' about consistency and not about saving storage space? The new node should have gotten its appropriate amount according to the ring responsibility (i.e., tokens). I'm not sure why a new node would get more than its fair share (according to tokens) of data though. There is one extreme case which would be if the cluster has seen lots of writes in degraded states so that there is a lot of data around the cluster that has not yet reached their full replica sets. A repair on a new node might make the new node be the only one that has all the data it should have... but you'd have to have written data at low consistency level during pretty shaky periods for this to have a significant effect (especially if hinted handoff is turned on). -- / Peter Schuller
