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The vnodes-to-tablets migration creates tablet maps that mirror the
vnode layout: one tablet per vnode, preserving token boundaries and
replica placement. However, due to tablet restrictions, the migration
requires vnode tokens to be a power of two and uniformly distributed
across the token ring.
In practice, this restriction is too limiting. Real clusters use
randomly generated tokens and a node's token assignment is immutable.
To solve this problem, prior work (01fb97ee78) has been done to relax
the tablet constraints by allowing arbitrary tablet boundaries, removing
the requirement for power-of-two sizing and uniform distribution.
This patch leverages the relaxed tablet constraints to enable tablet map
creation from arbitrary vnode tokens:
* Removes all token-related constraints.
* Handles wrap-around vnodes. If a vnode wraps (i.e., the highest vnode
token is not `dht::token::last()`), it is split into two tablets:
- (last_vnode_token, dht::token::last()]
- [dht::token::first(), first_vnode_token]
The migration ops guide has been updated to remove the power-of-two
constraint.
Signed-off-by: Nikos Dragazis <nikolaos.dragazis@scylladb.com>
