I wanted to use a RAM disk for my MySQL data files when running my tests for a Rails project. I succeeded, but it only cut 2 seconds off of my ~30 second test suite for my functional tests, and didn't cut any time off of my unit tests. But nevertheless, here is how I achieved this, in case it is useful to others. All of this is on OS X, using MySQL 5 from Macports.
What we are doing is creating a RAM disk, and then creating a seperate MySQL instance that stores its data on that RAM disk.
BASH:
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# Create the RAM disk
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hdid -nomount ram://52428800
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newfs_hfs /dev/disk1
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mkdir /tmp/ramdisk1
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mount -t hfs /dev/disk1 /tmp/ramdisk1
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# Initialize the MySQL environment
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mysql_install_db5 --datadir=/tmp/ramdisk1
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# Start the MySQL server
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/opt/local/libexec/mysqld --basedir=/opt/local --datadir=/tmp/ramdisk1 --pid-file=/tmp/mysql_memory_localhost.pid --port=10000 --socket=/tmp/mysql_memory.sock
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# Create your test database
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mysqladmin5 --socket=/tmp/mysql_memory.sock -uroot create projectname_test
Now in your database.yml file, add socket: /tmp/mysql_memory.sock to the test database configuration.
(thanks to this tutorial for how-to create a RAM disk in OS X)
Another useful trick is the MEMORY engine:
http://dev.mysql.com/doc/refman/5.0/en/memory-storage-engine.html
(Not that "MEMORY tables cannot contain BLOB or TEXT columns."). This may make them useless for you.
Yep, I considered that too. There's even a rails plugin to use sqlite3 memory tables. But i'd rather run my tests closer to the environment in which my application will be running, with transactions, etc. In fact, some of my tests test race conditions, so transactions are required.