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From:Rafał Radecki Date:January 8 2013 9:23am
Subject:Percona 5.5 semisynchronous replication.
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Hi all.

I have installed a setup in which I use semisync replication. On master I have:
mysql> show status like '%rpl%';
+--------------------------------------------+-------------+
| Variable_name                              | Value       |
+--------------------------------------------+-------------+
| Rpl_semi_sync_master_clients               | 1           |
| Rpl_semi_sync_master_net_avg_wait_time     | 1389        |
| Rpl_semi_sync_master_net_wait_time         | 159296355   |
| Rpl_semi_sync_master_net_waits             | 114643      |
| Rpl_semi_sync_master_no_times              | 0           |
| Rpl_semi_sync_master_no_tx                 | 0           |
| Rpl_semi_sync_master_status                | ON          |
| Rpl_semi_sync_master_timefunc_failures     | 0           |
| Rpl_semi_sync_master_tx_avg_wait_time      | 33380       |
| Rpl_semi_sync_master_tx_wait_time          | 77877106    |
| Rpl_semi_sync_master_tx_waits              | 2333        |
| Rpl_semi_sync_master_wait_pos_backtraverse | 43          |
| Rpl_semi_sync_master_wait_sessions         | 0           |
| Rpl_semi_sync_master_yes_tx                | 114667      |
| Rpl_semi_sync_slave_status                 | ON          |
| Rpl_status                                 | AUTH_MASTER |
+--------------------------------------------+-------------+
So semisync replication is enabled and working and there have been no
switches to async replication since mysql start.
But at the same time I have on the slave:

Seconds_Behind_Master: 364

And I wonder how is it possible?
As far as I understand semisync replication it works that way:
- master starts executing the query
- master sends the query to slave
- master starts the commit and waits for rpl_semi_sync_master_timeout
for ack from one of the slaves
So if I have rpl_semi_sync_master_timeout=5000 (5s) and semisync
replication is working and it has not switched to async replication
why is it possible on the slave to fall behind the master?

Best regards,
Rafal.
Thread
Percona 5.5 semisynchronous replication.Rafał Radecki8 Jan
  • Re: Percona 5.5 semisynchronous replication.Johan De Meersman8 Jan