Bucket "..." rebalance does not seem to be swap rebalance

[Lasse Schou]

Hi,

I have a Couchbase cluster (Community 2.0.1) with 5 machines, and I've tried this situation twice:

- One node fails over

- I click "Add back"

- I start rebalance

- A full rebalance starts, and in the log it says "Bucket "[bucket-name]" rebalance does not seem to be swap rebalance"

In the docs it says "When a node has failed, removing it and adding a replacement node, or adding the node back, will be treated as swap rebalance." (link: http://www.couchbase.com/docs/couchbase-manual-2.0/couchbase-admin-tasks-addremove-rebalance-swap.html)

Now my question is, why isn't a swap balance happening in this case?

[Matt Ingenthron]

On 5/12/13 11:59 AM, "Lasse Schou" <lasse...@gmail.com> wrote:

In the docs it says "When a node has failed, removing it and adding a replacement node, or adding the node back, will be treated as swap rebalance." (link: http://www.couchbase.com/docs/couchbase-manual-2.0/couchbase-admin-tasks-addremove-rebalance-swap.html)

Now my question is, why isn't a swap balance happening in this case?

I believe the docs are referring to a situation where you add a node at the time you trigger a failover, followed by a subsequent rebalance.  While it may not be considered a swap rebalance, it's similar in that data movement is minimized.

Consdider nodes in a cluster:

A B C

If I want to remove B and add D, I can do it in a "swap rebalance":

pre-rebalance: A B C

post-rebalance: A D C

(all vbuckets on B move to D)

This would be done by adding D and removing B before starting the rebalance.

Hope that helps,

Matt

-- 

Matt Ingenthron

Couchbase, Inc.

[Lasse Schou]

Hi Matt,

Ahh, so the docs are not talking about a failed over node, but rather a failed node that is removed, not failed over.

So can you confirm that if a node is auto-failed-over, there's no way to use a swap rebalance when adding the server back?

Thanks again, Lasse

2013/5/12 Matt Ingenthron <ma...@couchbase.com>

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[Aliaksey Kandratsenka]

On Sun, May 12, 2013 at 12:16 PM, Lasse Schou <lasse...@gmail.com> wrote:

Hi Matt,

Ahh, so the docs are not talking about a failed over node, but rather a failed node that is removed, not failed over.

So can you confirm that if a node is auto-failed-over, there's no way to use a swap rebalance when adding the server back?

Technically speaking, swap rebalance is when you swap one (or few) member nodes with same number of  brand new nodes. Failed over nodes are not full members. Because they don't have any active vbuckets left after fail over.

But we do detect failover-add-back (with original node or old node, doesn't matter) and we do generate vbucket map that's optimal. I.e. all vbuckets that were failed over are simply moved back.

So docs are only incorrect by calling all those cases swap rebalance.