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Branimir Lambov commented on CASSANDRA-7032: -------------------------------------------- I can't think of a way to do such a reduction on an existing DC. Adding a new node with the same number of vnodes should improve the distribution of tokens within the DC, but adding one with e.g. 8 in a 256-vnode DC will just give it 32-times smaller ownership. Trying to gradually transition will involve a period of severely overloading one or more of the non-converted nodes. It looks like DC-by-DC is you best bet, unfortunately. > Improve vnode allocation > ------------------------ > > Key: CASSANDRA-7032 > URL: https://issues.apache.org/jira/browse/CASSANDRA-7032 > Project: Cassandra > Issue Type: Improvement > Reporter: Benedict > Assignee: Branimir Lambov > Labels: performance, vnodes > Fix For: 3.0 alpha 1 > > Attachments: TestVNodeAllocation.java, TestVNodeAllocation.java, > TestVNodeAllocation.java, TestVNodeAllocation.java, TestVNodeAllocation.java, > TestVNodeAllocation.java > > > It's been known for a little while that random vnode allocation causes > hotspots of ownership. It should be possible to improve dramatically on this > with deterministic allocation. I have quickly thrown together a simple greedy > algorithm that allocates vnodes efficiently, and will repair hotspots in a > randomly allocated cluster gradually as more nodes are added, and also > ensures that token ranges are fairly evenly spread between nodes (somewhat > tunably so). The allocation still permits slight discrepancies in ownership, > but it is bound by the inverse of the size of the cluster (as opposed to > random allocation, which strangely gets worse as the cluster size increases). > I'm sure there is a decent dynamic programming solution to this that would be > even better. > If on joining the ring a new node were to CAS a shared table where a > canonical allocation of token ranges lives after running this (or a similar) > algorithm, we could then get guaranteed bounds on the ownership distribution > in a cluster. This will also help for CASSANDRA-6696. -- This message was sent by Atlassian JIRA (v6.4.14#64029) --------------------------------------------------------------------- To unsubscribe, e-mail: commits-unsubscr...@cassandra.apache.org For additional commands, e-mail: commits-h...@cassandra.apache.org