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  1. Couchbase Documentation
  2. DOC-9850

Document the limitation of server group data placement with respect to replica count in Couchbase Server 7.1

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Details

    • Bug
    • Status: Closed
    • Critical
    • Resolution: Fixed
    • Neo
    • Neo
    • architecture
    • None
    • 1

    Description

      As described in MB-50712, in 7.1 if there are at least two server groups the number of replicas placed for a bucket will be at most one less than the number of server groups. An example will probably help. E.g.

      • Say there are 2 server groups with 3 KV nodes each
      • A bucket is created with 2 replicas

      In 7.1 we will only place 1 replica for the bucket and we will not place the replica in a server group where the active vbucket is placed. (This is the work that is done for MB-43012.)

      We should update the Server Group page in the docs (https://docs.couchbase.com/server/current/learn/clusters-and-availability/groups.html#server-groups-and-vbuckets) to describe this limitation. We are working on addressing the limitation.

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            tony.hillman Tony Hillman added a comment -

            Does this mean that in 7.1, the diagram in https://docs.couchbase.com/server/current/learn/clusters-and-availability/groups.html#vbucket-distribution-across-unequal-groups is inaccurate? The diagram shows unequal server groups, with group 2 maintaining both the actives and the replicas for its 'extra' node, Server 9. In this situation, in 7.1, with two server groups, and only one replica therefore created, do the replica vBuckets for Server 9 get placed in group 1 instead? Thanks.

            tony.hillman Tony Hillman added a comment - Does this mean that in 7.1, the diagram in https://docs.couchbase.com/server/current/learn/clusters-and-availability/groups.html#vbucket-distribution-across-unequal-groups is inaccurate? The diagram shows unequal server groups, with group 2 maintaining both the actives and the replicas for its 'extra' node, Server 9. In this situation, in 7.1, with two server groups, and only one replica therefore created, do the replica vBuckets for Server 9 get placed in group 1 instead? Thanks.
            dfinlay Dave Finlay added a comment -

            Hi Tony - yes, indeed, that diagram needs to be updated. With the delivery of MB-43012, a replica will always be placed in a different server groups from the server group of the active even when the server groups have unequal sizes. I understand that Ray Offiah and Shivani Gupta have worked on the updates to this page – though I don't see any updates on the staging site as yet.

            dfinlay Dave Finlay added a comment - Hi Tony - yes, indeed, that diagram needs to be updated. With the delivery of MB-43012 , a replica will always be placed in a different server groups from the server group of the active even when the server groups have unequal sizes. I understand that Ray Offiah and Shivani Gupta have worked on the updates to this page – though I don't see any updates on the staging site as yet.

            Ray Offiah  I had reviewed the following pull request for unequal server groups:

            https://github.com/couchbase/docs-server/pull/2300

            Why haven't the above changes been merged into the staging docs website? I don't see anything there, unless I am looking at the wrong link.

            shivani.gupta Shivani Gupta added a comment - Ray Offiah   I had reviewed the following pull request for unequal server groups: https://github.com/couchbase/docs-server/pull/2300 Why haven't the above changes been merged into the staging docs website? I don't see anything there, unless I am looking at the wrong link.
            ray.offiah Ray Offiah added a comment -

            No, the link is correct; but it's sitting on a branch derived from 7.0 by the looks of it. I need to move the changes to a new PR on 7.1 otherwise it won't merge.

            ray.offiah Ray Offiah added a comment - No, the link is correct; but it's sitting on a branch derived from 7.0 by the looks of it. I need to move the changes to a new PR on 7.1 otherwise it won't merge.

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              ray.offiah Ray Offiah
              dfinlay Dave Finlay
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