MySQL Engines - MyISAM vs Innodb
MySQL supports several different types of Table Engines also known as "Table Types". A database can have its tables being a mix of different table engine types or all of the same type. Here is more information on each of the different types of table engines that MySQL offers:
The two most commonly used on most Cloud Sites MySQL servers use Innodb and MyISAM engines.
The purpose of this document is to briefly cover the two types and identify which ones are more recommended under what circumstances in the Cloud Sites environment. Please note that the purpose of this document is however not to go over a performance comparison of each of the two engine types as far as comparing via running specific sql test benchmarks, which if you are interested are well done on the two links below:
MyISAM is the default table engine type for MySQL 5.0 but the Cloud Sites environment defaults the storage engine to Innodb. In other words Cloud Sites is partial to Innodb if you do not explicity specify your engine type in your table DDL. We have also tuned the database servers to generally perform best with using the Innodb Engine type.
MyISAM vs Innodb - Quick comparison Table:
|Not *ACID compliant and non-transactional||*ACID compliant and hence fully transactional with ROLLBACK and COMMIT and support for Foreign Keys|
|MySQL 5.0 Default Engine||Rackspace Cloud Default Engine|
|Offers Compression||Offers Compression|
|Requires full repair/rebuild of indexes/tables||Auto recovery from crash via replay of logs|
|Changed Db pages written to disk instantly||Dirty pages converted from random to sequential before commit and flush to disk|
|No ordering in storage of data||Row data stored in pages in PK order|
|Table level locking||Row level locking|
- ACID - Atomicity, Consistency, Isolation, Durability (read more on it here: http://en.wikipedia.org/wiki/ACID)
If you need to see further details on each of the two engine types, please refer to the following MySQL documentations:
- Innodb Storage Engine: http://dev.mysql.com/doc/refman/5.0/en/innodb-storage-engine.html
- MyISAM Storage Engine: http://dev.mysql.com/doc/refman/5.0/en/myisam-storage-engine.html
When MyISAM tables are seen to be mostly useful?
There can be several other reasons that fit your requirement for choosing the MyISAM engine. For example reads can be faster on MyISAM vs Innodb despite what the general claims on the above two links when MyISAM table has fixed (not dynamic) row size i.e. when it uses more CHARs for example versus VARCHARs. Still there could be other reasons besides this why you choose or have chosen MyISAM over Innodb. Another reason why you may have chosen MyISAM over Innodb is perhaps due to the fact that Innodb must perform additional checks owing to its ACID compliant nature - so for example a FK check needs to be checked which could potentially cause an operational overhead. Unless you have benchmarked this to be the case, I would not recommend you believing this to be the case as default as per the links above, you may find out otherwise.
When we have seen conversion of table engine from MyISAM to Innodb as being most beneficial?
- If you need ACID compliance and need your db to be transactional then choosing Innodb is an obvious choice and you ought to make the necessary conversion including adding any FK constraints, etc if you need them.
- If you are not disproportionately read-only heavy and are doing a mix of reads (not requiring full text indexing) and writes then we do recommend that you go with Innodb.
- Most commonly we have observed that MyISAM tables would rather be converted to Innodb when you face frequent table lock escalations for long periods of time.
- If a read is slow or hasn't completed and a read/write is waiting on the first read to finish then the MyISAM table referenced in the read is held in a locked state till the resultset is made available to the query. This also causes a rise in the load average on the server and slows your site down. During this time no reads or writes can complete ofcourse as MyISAM only has table-level locking.
So to summarize, the queries that are victims of lock escalations under heavy but slow reads would do much better as a table converted to Innodb.
How to change your table Engine type from MyISAM to Innodb?
You do so by simply issuing the "ALTER TABLE" DDL statement:
ALTER TABLE <table-name> ENGINE=INNODB;
Below is a step by step process for altering a table in PHPMyAdmin:
- Log into the PHPMyAdmin utility through your control panel. If you are unsure how, please see Working with a MySQL database for instruction on how to login to PHPMyAdmin.
- Select the database which contains the Table-Name
- Click on the SQL tab
- Paste in the query provided above. Be sure to replace table-name with the correct name of your table
- Click the GO button.
Note: A MyISAM table that is using FULL TEXT Indexing can not be converted to an Innodb Table Engine type.
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