"Clusters with a large number of nodes, or a low number of vnodes, and a
high number of shards, or a combination, suffer from an aliasing problem:
both vnodes and intra-node sharding consider the most significant bits
to select the owning node and owning shard respectively. Since the same
bits are used for both, a low number of vnodes leads to some shards
being overcommitted relative to others.
This series fixes the problem by sharding on bits 0:47 of the token
(murmur3 partitioner only), leaving the most significant 12 bits for
vnodes. Simulation shows that this value provides reasonable sharding
for 100-node, 30-shard clusters.
In order to prevent re-sharding sstables on each boot, token ranges for
the range are stored in a new sub-component of the sstable Statistics
component. With the default 12 ignored bits we have 4096 token ranges
for non-Level-compacted SSTables, which takes some space but is still
reasonable.
Fixes #1277."
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