Q: Can you go directly MySQL 5.0 to 5.5 for MyISAM tables?
MyISAM have not been getting any significant development since MySQL 4.1, so in-place upgrade of MySQL 5.0 to MySQL 5.5 should be rather safe from data standpoint. There are still possibilities for sorting order and data type related changes, so you still need to run mysql_upgrade to check if any tables need to be rebuilt for new versions. Note this only applies to the data part of upgrade, you still need to keep into account possible query changes, replication etc.
Q: Is it possible to move MyISAM to Innodb through plain ALTER TABLE statements?
Yes. This is exactly how it is done in many cases. There is really no gain in dumping table via mysqldump and loading it back. If performance is of importance I would recommend using Percona Server with enabled expand_fast_index_creation feature, as I just wrote this option can help creating tables a lot faster. You can also use pt-online-schema-change tool for online migration.
Q: Should we change ROW replication to MIXED during Upgrade?
I prefer simple solutions when possible, which is either using STATEMENT or ROW replication. MIXED mode means you will have to deal with STATEMENT and ROW replication issues at the same time. I would only recommend MIXED replication mode if you have something substantial to gain.
Q: How would Upgrade work if Old Server has Antelope format and New one is Barracuda?
It should not be the problem. Innodb internal format is rather low level and it rarely impacts application functionality wise. As long a you go over normal upgrade checks you should be fine.
Q: What are the key concerns upgrading MySQL 4.1 to MySQL 5.5?
This is jumping over 2 Major versions and many years of development so it is the road less traveled and something not really tested well. One of specific issues might be making MySQL Replication to work. You might need to inject MySQL 5.0 in between so it will convert binary log events, as described here yet it does not guaranty replication will work for any application. There is just too many events. In the worse case you might need to do staged upgrades for your replicated setup or plan some downtime for upgrade.
Q: Upgrading to MySQL 5.5 do we really need all tables changed to Innodb and character set set to utf8?
No. MySQL 5.5 continues to support MyISAM tables, so you can continue use them as needed. I would evaluate changing though as MyISAM tables are on track to be depreciated in future MySQL versions also MyISAM tables are essentially at the state they were in MySQL 4.1 while Innodb storage engine got dramatic performance improvements and is getting better in each new release. Also note the default storage engine changed to Innodb in MySQL 5.5, meaning you might need to set storage_engine=MYISAM if you’re relaying on tables to be created as MyISAM by default. Regarding setting character set to utf8 – this is not needed as MySQL 5.5 continues to support various character sets same as previous MySQL versions. Moreover utf8 character set continues to cause significant performance overhead so you should only use it in case you’re storing multi language data.
Q: What to do if Query Execution Plan changes for the worst during Upgrade?
This is one of the big challenges with upgrade which can’t be resolved in all cases – sometimes queries just need to be changed or supplemented with hints to get to old execution plans. In many cases though you can get to former execution plan either by updating statistics (ANALYZE TABLE) or disabling some of the optimization/statistics related features in new version. Innodb statistics can especially be the problem as they can be very volatile and sometimes it is hard to get them to be same as in old version.
Q: What is about running 2 instances of MySQL with different versions to test upgrade?
This may or may not be good idea. In small environments when we often care more about functionality than performance this can be great way to validate the application is working without significant extra expenses. It also can be used for testing if hardware if there is little extra hardware available. If you’re doing performance testing though you need to ensure only one
instance is running at the same time so other has full resources (including memory) available. Another thing to mention having multiple instances of different MySQL versions on the same box typically means you will install them from binary distribution instead of packages (most typically used in production) which adds the difference compared to your production use. If resources allow I prefer to have upgrade testing done in the environment which is maximally close to production.
Q: How safe is it to upgrade from MySQL 5.5 to Percona Server 5.5
Compatibility is our highest priority with Percona Server, so it should be good as drop in replacement with no data conversion needed. You can also rollback easily unless you have enabled any special Percona Server features which are marked as making database binary incompatible with MySQL. Our focus on compatibility is indeed so high there are number of great features which are disabled by default to make sure you do not start relying on them by accident, so make sure to check out our Documentation to get most our of your Percona Server install.
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