Comment 1 for bug 1663761

Confusion came from the point that our document is bit stale on this front.

I found a clear documentation in codership and I am attaching the same.
It clarifies all the details including limitations.

http://galeracluster.com/documentation-webpages/mysqlwsrepoptions.html#wsrep-sst-donor-rejects-queries

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wsrep_sst_donor_rejects_queries

Defines whether the node rejects blocking client sessions on a node when it is serving as a donor in a blocking state transfer method, such as mysqldump and rsync.

Command-line Format --wsrep-sst-donor-rejects-queries
System Variable Name: wsrep_sst_donor_rejects_queries
Variable Scope: Global
Dynamic Variable:
Permitted Values Type: Boolean
Default Value: OFF
Support Introduced: 1
This parameter determines whether the node rejects blocking client sessions while it is sending state transfers using methods that block it as the donor. In these situations, all queries return the error ER_UNKNOWN_COM_ERROR, that is they respond with Unknown command, just like the joining node does.

Given that a State Snapshot Transfer is scriptable, there is no way to tell whether the requested method is blocking or not. You may also want to avoid querying the donor even with non-blocking state transfers. As a result, when this parameter is enabled the donor node rejects queries regardless the state transfer and even if the initial request concerned a blocking-only transfer, (meaning, it also rejects during xtrabackup).

Note Warning: The mysqldump state transfer method does not work with this setting, given that mysqldump runs queries on the donor and there is no way to differentiate its session from the regular client session.

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