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Re: RFC: new replication feature "per-engine mysql.gtid_slave pos"


Ian Gilfillan <ian@xxxxxxxxxxx> writes:

> In order to document this properly (and in response to

Great! Thanks for looking into this.

> https://jira.mariadb.org/browse/MDEV-13236) , I want to clarify a few
> things:

The main documentation is here, I'm sure it can be improved or made more


> As I understand it:
> * the variable is dynamic, so changes becomes effective immediately

Yes. Note that changing it requires that all slaves are stopped. So in that
sense, it only takes effect once slaves are running again.

> * tables are created immediately whenever a transaction is seen from a
> specified engine (listed in the variable)

Yes, for some value of "immediately". As mentioned in
https://mariadb.com/kb/en/mariadb/mysqlgtid_slave_pos-table/, the creation
is asynchroneous.

What this means is that, the need to create a table is discovered inside
record_gtid(), which is deep in the replication of the transaction when it
is time to commit it. It is not a good place to try to create a table. So
the transaction continues without the new table, and the table creation is
started in a background thread. So most likely the table will only be
available a bit later, after that transaction (and possibly a few other)
have been replicated.

Usually, the table should be created in a fraction of a second. Also note
that thanks to two-phase commit, the system is behaving correctly (in terms
of crash safety and consistency) also for the transactions that complete
prior to the new table being available.

> * the manual creation of the tables was an initial design suggestion,
> and there should be no need to ever create a table manually.

Yes, generally there should be no need to ever create a table manually. The
initial table is created by mysql_install_db (or mysql_upgrade), and
--gtid-pos-auto-engines can create any other tables required.

The one situation I can think of where manual action makes sense is as
explained in the documentation, in a system using primarily MyISAM, but the
database was created as InnoDB. In that case it can make sense to ALTER the
table to MyISAM (but not to create a table manually).

> I don't have 10.3.1 running yet to test, but some questions:
> * what happens if slave threads are not stopped before changing the
> variable? Just delay the creation of the table, or can something
> unpleasant happen from a table being created in the middle of
> something?

As explained in the documentation, this results in an error that slaves must
be stopped before changing the variable, similar to many other replication

> * what happens if a storage engine is later removed from the variable?
> If the table still exists, I presume the existence of the table takes
> priority and the table is still used - the variable is just used to
> check the existence of a table and create it if needs be, not
> determine its use?

Yes. The variable only concerns the _creation_ of the table. The _use_ of
the table is automatic whenever the table is there.

> * what benefit does the variable being empty (the default) provide? I
> saw the comment about the principle of least surprise, but in what way
> could it being populated and the tables existing be negative? In other
> words, why would one not simply recommend setting this immediately?

As I wrote before, I do not have any strong opinions on this.

The feature is designed to work well in all cases. Generally a default of
--gtid-pos-auto-engines=innodb,tokudb,rocksdb should be beneficial. It was
always a sore spot in the original GTID implementation that there was no
really good answer to which engine the mysql.gtid_slave_pos table should be
created with, and it has caused real problem as well. If the
mysql.gtid_slave_pos table was created as MyISAM, and
--gtid-pos-auto-engines=innodb,tokudb,rocksdb set as default, then there
will be a decent solution to this issue.

There are also some arguments in favour of an empty default, at least
initially until more testing has been done. MariaDB replication is extremely
complex code, there can be bugs. As you say, there is the principle of least
surprise. New tables magically springing into life during replication does
have some element of unpleasant surprise. And do not forget that this
feature is mostly/only useful to users that want to use TokuDB and RocksDB
(both still relatively new), _and_ at the same time use InnoDB, in a
performance-critical setting. Not a common usecase.

Technically, setting a non-empty default is just putting the right default
in sql/mysqld.cc, the code is already written to handle it. The current
empty default is based on the feedback from the users who took an interest
in the feature so far.

> * If there is a benefit to it being empty (and therefore no multiple
> tables created), will there be a mechanism to remove a table beyond
> manually deleting?

There is currently no such mechanism, and there should not be a need to
remove a table, I think.

There _is_ code that tries to correcly handle if a table is manually
removed. Such removal is best done with slave threads stopped. And the user
would be responsible that the rows in the table are either no longer needed,
or alternatively that they are migrated to the other tables. Generally, it
should not be recommended to ever do such manual removal, just like it is
not a good idea to manually remove or otherwise mess with other tables in
the mysql schema. That is why it is not documented - just like the effect
of removing other mysql.* tables is not documented.

I hope this helps. Feel free to ask again if you have more questions or if
something was unclear, and thanks for helping improve the documentation.

 - Kristian.

> On 03/07/2017 15:15, Kristian Nielsen wrote:
>> I have now pushed the code to 10.3. It should appear in an upcoming MariaDB
>> 10.3.1 release, IIUC.
>> Following the discussion so far, the default for --gtid-pos-auto-engines is
>> currently empty. It can be easily changed later (eg. to
>> innodb,tokudb,rocksdb) simply by changing the default value in
>> sql/mysqld.cc. The code will allow this default setting even if eg. tokudb
>> or rocksdb is not enabled.
>> (There might be a need to adjust a couple test cases that specially
>> manipulates mysql.gtid_slave_pos in devious ways).
>> Let me know in case of any problems or if there are comments/questions.
>>   - Kristian.
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