[Commits] fb619954867: MDEV-15010: Wrong Seconds_Behind_Master when only starting the SQL_Thread

andrei.elkin at pp.inet.fi andrei.elkin at pp.inet.fi
Tue Jul 16 21:01:19 EEST 2019


Sujatha, howdy.

Thanks for a good piece of work!
The fact that it's actually an upstream contribution only pleases more :-),
and to that regard we need a commit id to refer to (and to the people
behind that work :-)).

> revision-id: fb619954867232b3a733213a2703b2a425dff8e9 (mariadb-5.5.64-19-gfb619954867)
> parent(s): 7a7d9904e12335ee8b1eea9671138b3c469a3829
> author: Sujatha
> committer: Sujatha
> timestamp: 2019-06-19 14:48:18 +0530
> message:
>
> MDEV-15010: Wrong Seconds_Behind_Master when only starting the SQL_Thread
>
> Problem:
> =======
> When I run replication normally, I get a value in "Seconds_Behind_Master" from
> "SHOW SLAVE STATUS". However, when I run only the SQL Thread (on local relay
> logs that have been downloaded previously), I have "NULL" in
> "Seconds_Behind_Master". I would expect to have a numeric value.
>
> Fix:
> ===
> Implemented following changes.
> case 1: "Seconds_Behind_Master" shows 0, when SQL thread is in sync with IO
>         thread and IO thread is running.
> case 2: "Seconds_Behind_Master" reports NULL, when SQL thread is in sync with
>         IO thread and IO thread is stopped.
> case 3: "Seconds_Behind_Master" reports NULL, when SQL thread is stopped while
>         IO thread is up and running.
> case 4: "Seconds_Behind_Master" reports a valid numerical value when IO thread
>         is stopped and SQL thread is consuming existing relay log.

I have a question to the case 4 in the tests actually.
>
> ---
>  .../suite/rpl/r/rpl_seconds_behind_master.result   | 40 ++++++++++
>  .../suite/rpl/t/rpl_seconds_behind_master.test     | 92 ++++++++++++++++++++++
>  sql/slave.cc                                       | 82 ++++++++++++-------
>  3 files changed, 187 insertions(+), 27 deletions(-)
>
> diff --git a/mysql-test/suite/rpl/r/rpl_seconds_behind_master.result b/mysql-test/suite/rpl/r/rpl_seconds_behind_master.result
> new file mode 100644
> index 00000000000..1c0ce7c1b19
> --- /dev/null
> +++ b/mysql-test/suite/rpl/r/rpl_seconds_behind_master.result
> @@ -0,0 +1,40 @@
> +include/master-slave.inc
> +[connection master]
> +CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
> +******************************************************************
> +* Case1: SQL thread is in sync with IO thread and IO thread is UP
> +*        Seconds_Behind_Master should be '0'.
> +******************************************************************
> +include/assert.inc [Seconds_Behind_Master should be 0]
> +******************************************************************
> +* Case2: SQL thread is in sync with IO thread and IO thread is
> +*        stopped.
> +*        Seconds_Behind_Master should be 'NULL'.
> +******************************************************************
> +include/stop_slave_io.inc
> +include/assert.inc [Seconds_Behind_Master should be NULL]
> +******************************************************************
> +* Case3: SQL thread is stopped while IO thread is up and running.
> +*        Seconds_Behind_Master should be 'NULL'.
> +******************************************************************
> +START SLAVE IO_THREAD;
> +include/wait_for_slave_io_to_start.inc
> +include/stop_slave_sql.inc
> +include/assert.inc [Seconds_Behind_Master should be NULL]
> +******************************************************************
> +* Case4: IO thread is stopped. SQL thread is consuming existing relay
> +*        log. Hence the Seconds_Behind_Master should have a numeric
> +*        value > 0
> +******************************************************************
> +INSERT INTO t1 VALUES (1,sleep(2));
> +INSERT INTO t1 VALUES (2,sleep(2));
> +include/sync_slave_io_with_master.inc
> +include/stop_slave_io.inc
> +START SLAVE SQL_THREAD;
> +include/wait_for_slave_sql_to_start.inc
> +include/assert.inc [Seconds_Behind_Master should be > 0]
> +START SLAVE IO_THREAD;
> +include/wait_for_slave_io_to_start.inc
> +******* CLEANUP *********
> +DROP TABLE t1;
> +include/rpl_end.inc
> diff --git a/mysql-test/suite/rpl/t/rpl_seconds_behind_master.test b/mysql-test/suite/rpl/t/rpl_seconds_behind_master.test
> new file mode 100644
> index 00000000000..6f9ed2939e1
> --- /dev/null
> +++ b/mysql-test/suite/rpl/t/rpl_seconds_behind_master.test
> @@ -0,0 +1,92 @@
> +# ==== Purpose ====
> +#
> +# Test verifies that Seconds_Behind_Master(SBM) values are as expected in various
> +# scenario listed below.
> +#
> +# case 1: SBM=0 if SQL thread is in sync with IO thread and IO thread is UP
> +# case 2: SBM=NULL if SQL thread is in sync with IO thread and IO thread is
> +#         stopped.
> +# case 3: SBM=NULL if SQL thread is stopped while IO thread is up and running.
> +# case 4: SBM > 0 and SBM != NULL if IO thread is stopped. SQL thread is
> +#         consuming existing relay log.
> +#
> +# ==== Implementation ====
> +#
> +# Steps:
> +#    1 - Create a table and sync it with slave and verify SBM value.
> +#    2 - Now stop IO thread and verify SBM
> +#    3 - Start IO thread, Stop SQL thread and verify SBM
> +#    4 - Execute slow INSERT statements with SLEEP(2) on master. Ensure that
> +#        the IO thread has read these INSERT statements and then bring it down.
> +#    5 - Start the SQL thread and verify that while it is trying to consume
> +#        these slow INSERT statements in relay log the SBM value is > 0.
> +#
> +# ==== References ====
> +#
> +# MDEV-15010: Wrong Seconds_Behind_Master when only starting the SQL_Thread.
> +
> +--source include/have_binlog_format_statement.inc
> +--source include/master-slave.inc
> +
> +--disable_query_log
> +CALL mtr.add_suppression("Unsafe statement written to the binary log using statement format since BINLOG_FORMAT = STATEMENT");
> +--enable_query_log
> +
> +CREATE TABLE t1 (a INT PRIMARY KEY, b INT);
> +--sync_slave_with_master
> +
> +-- echo ******************************************************************
> +-- echo * Case1: SQL thread is in sync with IO thread and IO thread is UP
> +-- echo *        Seconds_Behind_Master should be '0'.
> +-- echo ******************************************************************
> +--let $assert_cond= [SHOW SLAVE STATUS, Seconds_Behind_Master, 1] = 0
> +--let $assert_text= Seconds_Behind_Master should be 0
> +--source include/assert.inc
> +
> +-- echo ******************************************************************
> +-- echo * Case2: SQL thread is in sync with IO thread and IO thread is
> +-- echo *        stopped.
> +-- echo *        Seconds_Behind_Master should be 'NULL'.
> +-- echo ******************************************************************
> +--source include/stop_slave_io.inc
> +--let $assert_cond= "[SHOW SLAVE STATUS, Seconds_Behind_Master, 1]" = "NULL"
> +--let $assert_text= Seconds_Behind_Master should be NULL
> +--source include/assert.inc
> +
> +-- echo ******************************************************************
> +-- echo * Case3: SQL thread is stopped while IO thread is up and running.
> +-- echo *        Seconds_Behind_Master should be 'NULL'.
> +-- echo ******************************************************************
> +START SLAVE IO_THREAD;
> +--source include/wait_for_slave_io_to_start.inc
> +--source include/stop_slave_sql.inc
> +--let $assert_cond= "[SHOW SLAVE STATUS, Seconds_Behind_Master, 1]" = "NULL"
> +--let $assert_text= Seconds_Behind_Master should be NULL
> +--source include/assert.inc
> +
> +-- echo ******************************************************************
> +-- echo * Case4: IO thread is stopped. SQL thread is consuming existing relay
> +-- echo *        log. Hence the Seconds_Behind_Master should have a numeric
> +-- echo *        value > 0
> +-- echo ******************************************************************
> +--connection master
> +--disable_warnings
> +INSERT INTO t1 VALUES (1,sleep(2));
> +INSERT INTO t1 VALUES (2,sleep(2));
> +--enable_warnings
> +--source include/sync_slave_io_with_master.inc
> +--source include/stop_slave_io.inc
> +
> +START SLAVE SQL_THREAD;
> +--source include/wait_for_slave_sql_to_start.inc

There's a theoretical (and even though I think it would
hardly ever be practical) chance for the SQL thread to catch up
while for some concurrency reason the waiting would take longer that
it's naturally expected.

How about to

+ LOCK TABLES t1 WRITE
before START SLAVE SQL_THREAD
and unlock
> +--let $assert_cond= [SHOW SLAVE STATUS, Seconds_Behind_Master, 1] > 0
> +--let $assert_text= Seconds_Behind_Master should be > 0
> +--source include/assert.inc

after the assert check?

This measure would speak rather explicitly that the check is done just
before any first event has been processed by the SQL thread, and as you
prepare 2, then no synchronization is guaranteed.

Cheers,

Andrei

> +START SLAVE IO_THREAD;
> +--source include/wait_for_slave_io_to_start.inc
> +
> +--echo ******* CLEANUP *********
> +--connection master
> +DROP TABLE t1;
> +
> +--source include/rpl_end.inc
> diff --git a/sql/slave.cc b/sql/slave.cc
> index a8946c69d18..7b4b7bd857e 100644
> --- a/sql/slave.cc
> +++ b/sql/slave.cc
> @@ -2192,38 +2192,66 @@ bool show_master_info(THD* thd, Master_info* mi)
>      protocol->store(mi->ssl_cert, &my_charset_bin);
>      protocol->store(mi->ssl_cipher, &my_charset_bin);
>      protocol->store(mi->ssl_key, &my_charset_bin);
> -
>      /*
> -      Seconds_Behind_Master: if SQL thread is running and I/O thread is
> -      connected, we can compute it otherwise show NULL (i.e. unknown).
> +       The pseudo code to compute Seconds_Behind_Master:
> +       if (SQL thread is running)
> +       {
> +         if (SQL thread processed all the available relay log)
> +         {
> +           if (IO thread is running)
> +             print 0;
> +           else
> +             print NULL;
> +         }
> +         else
> +           compute Seconds_Behind_Master;
> +       }
> +       else
> +         print NULL;
>      */
> -    if ((mi->slave_running == MYSQL_SLAVE_RUN_CONNECT) &&
> -        mi->rli.slave_running)
> +    if (mi->rli.slave_running)
>      {
> -      long time_diff= ((long)(time(0) - mi->rli.last_master_timestamp)
> -                       - mi->clock_diff_with_master);
>        /*
> -        Apparently on some systems time_diff can be <0. Here are possible
> -        reasons related to MySQL:
> -        - the master is itself a slave of another master whose time is ahead.
> -        - somebody used an explicit SET TIMESTAMP on the master.
> -        Possible reason related to granularity-to-second of time functions
> -        (nothing to do with MySQL), which can explain a value of -1:
> -        assume the master's and slave's time are perfectly synchronized, and
> -        that at slave's connection time, when the master's timestamp is read,
> -        it is at the very end of second 1, and (a very short time later) when
> -        the slave's timestamp is read it is at the very beginning of second
> -        2. Then the recorded value for master is 1 and the recorded value for
> -        slave is 2. At SHOW SLAVE STATUS time, assume that the difference
> -        between timestamp of slave and rli->last_master_timestamp is 0
> -        (i.e. they are in the same second), then we get 0-(2-1)=-1 as a result.
> -        This confuses users, so we don't go below 0: hence the max().
> -
> -        last_master_timestamp == 0 (an "impossible" timestamp 1970) is a
> -        special marker to say "consider we have caught up".
> +         Check if SQL thread is at the end of relay log
> +         Checking should be done using two conditions
> +         condition1: compare the log positions and
> +         condition2: compare the file names (to handle rotation case)
>        */
> -      protocol->store((longlong)(mi->rli.last_master_timestamp ?
> -                                 max(0, time_diff) : 0));
> +      if ((mi->master_log_pos == mi->rli.group_master_log_pos) &&
> +          (!strcmp(mi->master_log_name, mi->rli.group_master_log_name)))
> +      {
> +        if (mi->slave_running == MYSQL_SLAVE_RUN_CONNECT)
> +          protocol->store(0LL);
> +        else
> +          protocol->store_null();
> +      }
> +      else
> +      {
> +        long time_diff = ((long)(time(0) - mi->rli.last_master_timestamp)
> +            - mi->clock_diff_with_master);
> +        /*
> +           Apparently on some systems time_diff can be <0. Here are possible
> +           reasons related to MySQL:
> +           - the master is itself a slave of another master whose time is ahead.
> +           - somebody used an explicit SET TIMESTAMP on the master.
> +           Possible reason related to granularity-to-second of time functions
> +           (nothing to do with MySQL), which can explain a value of -1:
> +           assume the master's and slave's time are perfectly synchronized, and
> +           that at slave's connection time, when the master's timestamp is read,
> +           it is at the very end of second 1, and (a very short time later) when
> +           the slave's timestamp is read it is at the very beginning of second
> +           2. Then the recorded value for master is 1 and the recorded value for
> +           slave is 2. At SHOW SLAVE STATUS time, assume that the difference
> +           between timestamp of slave and rli->last_master_timestamp is 0
> +           (i.e. they are in the same second), then we get 0-(2-1)=-1 as a result.
> +           This confuses users, so we don't go below 0: hence the max().
> +
> +           last_master_timestamp == 0 (an "impossible" timestamp 1970) is a
> +           special marker to say "consider we have caught up".
> +        */
> +        protocol->store((longlong)(mi->rli.last_master_timestamp ?
> +              max(0, time_diff) : 0));
> +      }
>      }
>      else
>      {
> _______________________________________________
> commits mailing list
> commits at mariadb.org
> https://lists.askmonty.org/cgi-bin/mailman/listinfo/commits


More information about the commits mailing list