[MDEV-13477] Seconds_Behind_Master is always 0 when running show slave status Created: 2017-08-09  Updated: 2017-09-11  Resolved: 2017-09-11

Status: Closed
Project: MariaDB Server
Component/s: Replication
Affects Version/s: 10.2.7
Fix Version/s: N/A

Type: Bug Priority: Major
Reporter: Michael Xu Assignee: Unassigned
Resolution: Incomplete Votes: 0
Labels: need_feedback
Environment:

CentOS 7.x x86_64



 Description   

Output mesages:

               Slave_IO_State: Waiting for master to send event
                  Master_Host: 172.x.xx.xxx
                  Master_User: replicate
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000239
          Read_Master_Log_Pos: 51505421
               Relay_Log_File: relay-bin.000002
                Relay_Log_Pos: 670901269
        Relay_Master_Log_File: mysql-bin.000237
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB: 
          Replicate_Ignore_DB: 
           Replicate_Do_Table: 
       Replicate_Ignore_Table: 
      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 0
                   Last_Error: 
                 Skip_Counter: 0
          Exec_Master_Log_Pos: 763661241
              Relay_Log_Space: 2206074748
              Until_Condition: None
               Until_Log_File: 
                Until_Log_Pos: 0
           Master_SSL_Allowed: No
           Master_SSL_CA_File: 
           Master_SSL_CA_Path: 
              Master_SSL_Cert: 
            Master_SSL_Cipher: 
               Master_SSL_Key: 
        Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
                Last_IO_Errno: 0
                Last_IO_Error: 
               Last_SQL_Errno: 0
               Last_SQL_Error: 
  Replicate_Ignore_Server_Ids: 
             Master_Server_Id: 3
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
                   Using_Gtid: No
                  Gtid_IO_Pos: 
      Replicate_Do_Domain_Ids: 
  Replicate_Ignore_Domain_Ids: 
                Parallel_Mode: optimistic
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: apply log event

Meanwhile, I can see thread Slave_SQL is busy, and the four Slave_worker threads are idle, not sure if this is a bug.



 Comments   
Comment by Elena Stepanova [ 2017-08-14 ]

There is a lot of confusion about what Seconds_Behind_Master is supposed to show. Please check the documentation in regard to the meaning of the value:
https://mariadb.com/kb/en/mariadb/show-slave-status/

Difference between the timestamp logged on the master for the event that the slave is currently processing, and the current timestamp on the slave. Zero if the slave is not currently processing an event. From MariaDB 10.0.23 and MariaDB 10.1.9, with parallel replication, seconds_behind_master is updated only after transactions commit.

If it doesn't correlate with your observations, please let us know.

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