W always stands for number of sync writes. N-W is the number of async writes. Note, N decides number of replicas. W only decides out of those N replicas, how many should be written synchronously before returning success of write to client. All writes always happen to a total of N nodes (W right away and the rest later) The higher the value of W the more sync writes and so more latency.
I might be wrong, but I think you cannot decide which of the N nodes will get the sync write. On a write-by-write basis, I think Cassandra needs the flexibility to decide on several parameters which W out of N nodes would it write synchronously. On Fri, Feb 18, 2011 at 3:48 PM, mcasandra <mohitanch...@gmail.com> wrote: > > I am still trying to understand how writes work. Is there any concept of sync > and async writes? For eg: > > If I want to have W=2 but 1 write as sync and the 2nd as async. > > Or say I want to have W=3 with networktopology with DC1 getting 1 sync write > + 1 async write and DC2 always getting async write. > > > -- > View this message in context: > http://cassandra-user-incubator-apache-org.3065146.n2.nabble.com/Async-write-tp6041440p6041440.html > Sent from the cassandra-u...@incubator.apache.org mailing list archive at > Nabble.com. >
