DB2 has a long history, having ran on IBM mainframes, and mid-range for decades. Now it easy to move your rails app using it.
The lovely thing about the latest version 9.5 DB2 is that you can use
it for free, if your using the DB2-ExpressC version. It has most all the features you will need, and the database size is not limited. You can add support, or add-ons easily.
The issue you will have, if you have a existing rails app, is moving your data from mysql, or other rails database into db2.
The idea solution for this is a rails plugin, called YamlDb.
In Rails 2.1 its as easy as:
script/plugin install git://github.com/adamwiggins/yaml_db.git
then update your database config file to point to db2:
The rake db:create does not work in db2, so:
db2 create database baseapp
And your ready to go.
Not that database names can only be 8 characters, and underlines and spaces may cause you problems. Otherwise, its painless.