复制问题

Question

We have a one master and two VIP slave database servers. We changed data type of column from VARCHAR(255) to TEXT on master. The application is currently configured to use master only for writing operations and configured slaves for reading operation. After changing the data type on master server using ALTER TABLE command the slave server becomes unresponsive. We are using Mariadb 10.0 [PROCESSES INFORMATION]

Id  User    Host    Db  Command Time(sec)   State   Info
-----------------------------------------------------------------------
203739  repl    slave1      Binlog Dump 75,143,121  Master has sent all binlog to slave; waiting for binlog to be updated   

203740  repl    slave2      Binlog Dump 75,143,121  Master has sent all binlog to slave; waiting for binlog to be updated

The slave instance becomes very slow due to slow queries. number of sessions: 1590 thread_pool_max_thread=500 Current value =648

After performing ALTER TABLE on Master server, it was replicating to slave server and in the same time number of sessions were get increased rapidly on slave server. I think slaves becomes unresponsive because of slow queries.

But I don't know why this queries became so slow and slaves got unresponsive. The DBA's saying that after executing ANALYZE TABLE command, the issue has been solved. But I don't understand why this happened because ANALYZE TABLE only update the statistic information. It would be helpful if anyone comment on this why it happened? How to avoid such issues in future.

Answer 1

There is one minor case where TEXT is slower than VARCHAR . When a SELECT needs to build a temporary table (often for sorting due to GROUP BY or ORDER BY ), it first tries to build a MEMORY table. But, TEXT and BLOB prevent it from using such, so it uses MyISAM instead. This is slower (but gets the job done).

I say this is a "minor case" because users rarely identify it with phrases like "very slow" and "becomes unresponsive". I would guess that a SELECT might run twice as slow.

Also, the ANALYZE TABLE discussion does not hold water. Again it may be coincidence, not causation.

So, the change to TEXT may be a 'red herring'. Instead, let's discover what is being slow by using the slowlog. See this for what I like to work from.