[CASSANDRA-7032] Improve vnode allocation

• Type: Improvement
• Status: Resolved
• Priority: Normal
• Resolution: Fixed
• Fix Version/s: 3.0 alpha 1
• Component/s: Feature/Lightweight Transactions
• Labels: LWT performance vnodes
• Assignee: Branimir Lambov
• Reporter: Benedict Elliott Smith

Authors

Branimir Lambov

Reviewers

Benedict Elliott Smith

• Created: 14/Apr/14 12:05
• Updated: 25/Oct/19 13:11
• Resolved: 23/Jun/15 13:39

Description

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.

Issue Links

• incorporates: CASSANDRA-6388 delay vnode selection if bootstrapping
• is related to: CASSANDRA-8338 Simplify Token Selection
• is related to: CASSANDRA-3810 reconsider rack awareness
• is required by: CASSANDRA-6696 Partition sstables by token range
• relates to: CASSANDRA-11139 New token allocator is broken