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  1. MariaDB Server
  2. MDEV-17516

Replication lag issue using parallel replication

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    Description

      Using parallel replication second behind master is wrongly reporting 0 when SQL thread is stopped and restarted long time after.

      This happen by design
      https://lists.launchpad.net/maria-developers/msg08958.html

      but is really a show stopper for most proxy that send traffic to such slave thinking it's in sync with master.

      My understanding is that slave_behind_master is computed after first commit so in this case the master is 2 days in advance and on a fresh restarted slave we get this

      |   30 | system user  |                      | tsce_unedic | Connect | 2211 | altering table                                                                 | OPTIMIZE TABLE `requetes` |    0.000 |

      And we can see wrong second behind master

              Seconds_Behind_Master: 0
      		 
                         Using_Gtid: Slave_Pos
      		 
                        Gtid_IO_Pos: 0-21-28557589
      		 
                      Parallel_Mode: conservative

      but on his master 

      gtid_current_pos       | 0-21-28570301

      A possible solution would be to update Seconds_Behind_Master by injecting a fake event in start slave with the max timestamp of all events read by the leader thread and send to to the worker threads .

      To reproduce :

      --source include/have_innodb.inc
      		 
      --source include/have_binlog_format_mixed.inc
      		 
      --let $rpl_topology=1->2
      		 
      --source include/rpl_init.inc
      		 
       
      		 
      # Test various aspects of parallel replication.
      		 
       
      		 
      --connection server_1
      		 
      ALTER TABLE mysql.gtid_slave_pos ENGINE=InnoDB;
      		 
       
      		 
      CREATE TABLE t1 (a INT PRIMARY KEY, b INT) ENGINE=InnoDB;
      		 
      --save_master_pos
      		 
       
      		 
      --connection server_2
      		 
      --sync_with_master
      		 
      --source include/stop_slave_sql_thread.inc
      		 
      SET GLOBAL slave_parallel_threads=4;
      		 
       
      		 
      --connection server_2
      		 
      --sync_with_master
      		 
      --source include/stop_slave.inc
      		 
      SET GLOBAL slave_parallel_threads=1;
      		 
       
      		 
      --connection server_1
      		 
      --disable_warnings
      		 
      INSERT INTO t1 VALUES (1, SLEEP(100));
      		 
      --wait 100s
      		 
      INSERT INTO t1 VALUES (1, SLEEP(1));
      		 
       
      		 
      --connection server_2
      		 
      --source include/start_slave.inc
      		 
      --let $status_items= Seconds_Behind_Master
      		 
      --source include/show_slave_status.inc
      		 
      --sync_with_master
      		 
      --let $status_items= Seconds_Behind_Master
      		 
      --source include/show_slave_status.inc

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          is duplicated by

          Bug - A problem which impairs or prevents the functions of the product. MDEV-29639 Seconds_Behind_Master is incorrect for Delayed, Parallel Replicas

          • Critical - Crashes, loss of data, severe memory leak.
          • Closed
          relates to

          Task - A task that needs to be done. MDEV-30458 Consolidate Serial Replica to Parallel Replica with 1 Worker Thread

          • Major - Major loss of function.
          • Open

          Bug - A problem which impairs or prevents the functions of the product. MDEV-31745 First Event After Starting a Delayed Parallel Replica Shows 0 Seconds_Behind_Master

          • Major - Major loss of function.
          • Open

          Bug - A problem which impairs or prevents the functions of the product. MDEV-7837 Seconds behind Master reports incorrect value when Parallel replication is used

          • Major - Major loss of function.
          • Stalled

          Bug - A problem which impairs or prevents the functions of the product. MDEV-32265 seconds_behind_master is inaccurate for Delayed replication

          • Major - Major loss of function.
          • Closed

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              bnestere Brandon Nesterenko
              stephane@skysql.com VAROQUI Stephane
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              Dates

                Created:
                Updated:

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