When a node joins a cluster, it gets (semi-)random tokens based on num_tokens value.
Total amount of vnodes is not fixed. I don’t remember top of my hat if num_tokens can be different on each node but whenever you add a node, new vnodes get “created”. Existing token ranges will be split and some range will be allocated for the new node and data is being replicated to the joining node. So if you have num_tokens set to a higher value like 16 or so, adding and removing a single node in a cluster is standard operation and although it causes some load on the cluster, it should be somewhat evenly distributed among other nodes. If you have just a single token per node then scaling up or down has a bit different effects due to balancing issues etc. So there is a reason why default num_tokens is 16 currently. Cheers, Hannu > On 15. Jun 2022, at 10.12, Luca Rondanini <luca.rondan...@gmail.com> wrote: > > ok, that makes sense, but does the partitioner add vnodes? is the number of > vnodes fixed in a cluster? > > On Wed, Jun 15, 2022 at 12:10 AM Hannu Kröger <hkro...@gmail.com > <mailto:hkro...@gmail.com>> wrote: > Hey, > > num_tokens is tokens per node. > > So in your case you would have 15 vnodes altogether. > > Cheers, > Hannu > > > On 15. Jun 2022, at 10.08, Luca Rondanini <luca.rondan...@gmail.com > > <mailto:luca.rondan...@gmail.com>> wrote: > > > > Hi all, > > > > I'm just trying to understand better how cassandra works. > > > > My understanding is that, once set, the number of vnodes does not change in > > a cluster. The partitioner allocates vnodes to nodes ensuring replication > > data are not stored on the same node. > > > > But what happens if there are more nodes than vnodes? If I set num_tokens > > to 3 and I have 5 servers? Unless the partitioner adds vnodes and moves > > data around but it seems an extremely expensive operation. I'm sure I'm > > missing something, I'm not quite sure what! :) > > > > Thanks, > > Luca > > >
