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)
