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  1. Documentation
  2. DOCS-10962

Document balancer 3.2 vs 3.4

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      We should write in one place the different behavior of the balancer in MongoDB 3.2 vs MongoDB 3.4

      3.2:

      • The balancer will balance only one collection at a time among the entire cluster.
      • Only one chunk is moved at a time.
      • It’s possible to run manually moveChunk operations on two different collections if they do not impact the same shards. Ex. for collection1 move a chunk from Shard0 to Shard1, for collection2 move a chunk from Shard2 to Shard3.

      3.4:

      • For the same collection is possible to have two migrations ongoing at the same time on two different couple of shards (ex Shard0->Shard1, Shard3->Shard4).
      • MongoDB can perform at most n/2 (rounded down) simultaneous chunk migrations.
      • The balancer is able to balance more than one collection at a time with the caveat that the auto-balancer will favor balancing one collection fully before looking for other collections. This means if you have 2 collections eligible for balancing, but collection1 has many chunks to be moved, it will always fill up the available shard pairs on each round and the second collection will not be picked up until collection1 is done balancing.
        For example, the following scenario is possible:
        + Shard0->Shard1 (migration for collection1)
        + Shard2->Shard3 (migration for collection1)
        + Shard4->Shard5 (migration for collection2)
        On the other hand if collection1 has chunks which needed to be balanced from Shard4 to Shard5, the entry for collection2 above would not be selected by the auto-balancer, but instead, some chunk from collection1 would be.
      • It’s possible to run manually moveChunk operations on two different collections if they do not impact the same shards. Ex. for collection1 move a chunk from Shard0 to Shard1, for collection2 move a chunk from Shard2 to Shard3.

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            Assignee:
            Unassigned Unassigned
            Reporter:
            renato.riccio Renato Riccio
            Participants:
            Last commenter:
            Anthony Sansone Anthony Sansone
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              Updated:
              Days since reply:
              3 years, 39 weeks, 4 days ago
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