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Bring a tablet that has lost a majority of replicas back online

If a tablet has permanently lost a majority of its replicas, it cannot recover automatically and operator intervention is required. If the tablet servers hosting a majority of the replicas are down (i.e. ones reported as "TS unavailable" by ksck), they should be recovered instead if possible.

Suppose a tablet has lost a majority of its replicas. The first step in diagnosing and fixing the problem is to examine the tablet's state using ksck: 

$ sudo -u kudu kudu cluster ksck --tablets=e822cab6c0584bc0858219d1539a17e6 master-00,master-01,master-02
Connected to the Master
Fetched info from all 5 Tablet Servers
Tablet e822cab6c0584bc0858219d1539a17e6 of table 'my_table' is unavailable: 2 replica(s) not RUNNING
  638a20403e3e4ae3b55d4d07d920e6de (tserver-00:7150): RUNNING
  9a56fa85a38a4edc99c6229cba68aeaa (tserver-01:7150): bad state
    State:       FAILED
    Data state:  TABLET_DATA_READY
    Last status: <failure message>
  c311fef7708a4cf9bb11a3e4cbcaab8c (tserver-02:7150): bad state
    State:       FAILED
    Data state:  TABLET_DATA_READY
    Last status: <failure message>

This output shows that, for tablet e822cab6c0584bc0858219d1539a17e6, the two tablet replicas on tserver-01 and tserver-02 failed. The remaining replica is not the leader, so the leader replica failed as well. This means the chance of data loss is higher since the remaining replica on tserver-00 may have been lagging. In general, to accept the potential data loss and restore the tablet from the remaining replicas, divide the tablet replicas into two groups:

  1. Healthy replicas: Those in RUNNING state as reported by ksck
  2. Unhealthy replicas

For example, in the above ksck output, the replica on tablet server tserver-00 is healthy while the replicas on tserver-01 and tserver-02 are unhealthy. On each tablet server with a healthy replica, alter the consensus configuration to remove unhealthy replicas. In the typical case of 1 out of 3 surviving replicas, there will be only one healthy replica, so the consensus configuration will be rewritten to include only the healthy replica. 

$ sudo -u kudu kudu remote_replica unsafe_change_config tserver-00:7150 <tablet-id> <tserver-00-uuid>

where <tablet-id> is e822cab6c0584bc0858219d1539a17e6 and <tserver-00-uuid> is the uuid of tserver-00, 638a20403e3e4ae3b55d4d07d920e6de.

Once the healthy replicas' consensus configurations have been forced to exclude the unhealthy replicas, the healthy replicas will be able to elect a leader. The tablet will become available for writes though it will still be under-replicated. Shortly after the tablet becomes available, the leader master will notice that it is under-replicated, and will cause the tablet to re-replicate until the proper replication factor is restored. The unhealthy replicas will be tombstoned by the master, causing their remaining data to be deleted.