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From "Jonathan Ellis (JIRA)" <>
Subject [jira] Updated: (CASSANDRA-762) Load balancing does not account for the load of the moving node
Date Thu, 04 Feb 2010 19:26:28 GMT


Jonathan Ellis updated CASSANDRA-762:

      Component/s: Core
    Fix Version/s: 0.6

As I said in IRC, a loadbalancing node doesn't compute its destination token until after all
its data has been transferred off.  (We *should* add a sleep until load is rebroadcast, but
that will only affect which node gets picked to pull half the range from.)  So if there is
a bug here it is in the code, not the algorithm.

Can you reproduce this?

> Load balancing does not account for the load of the moving node
> ---------------------------------------------------------------
>                 Key: CASSANDRA-762
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.5
>            Reporter: Stu Hood
>            Priority: Minor
>             Fix For: 0.6
> Given a node A (with load 10 gb) and a node B (with load 20 gb), running the loadbalance
command against node A will:
> 1. Remove node A from the ring
>   * Recalculates pending ranges so that node B is responsible for the entire ring
> 2. Pick the most loaded node
>   * node B is still reporting 20 gb load, because that is all it has locally
> 3. Choose a token that divides the range of the most loaded node in half
> Since the token calculation doesn't take into account the load that node B is 'inheriting'
from node A, the token will divide node B's load in half and swap the loads. Instead, the
token calculation needs to pretend that B has already inherited the 10 gb from node A, for
a total of 30 gb. The token that should be chosen falls at 15 gb of the total load, or 5 gb
into node B's load.

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