A good description on how to create your own fusion drive – useful now that SSD’s are getting very inexpensive.
CoreStorage is Apple’s volume manager that provides an interface between the OS and physical volumes on the drive. It was introduced in OS X Lion for its full-disk encryption options with FileVault 2, but in addition allows for volume spanning, where multiple drives in the system can be combined into one logical volume for the OS to use.
The CoreStorage volume-spanning feature not only increases a single volume’s storage beyond the capacity of a single drive, it also prioritizes storage to the primary drive in its logical volume. This means that if you use an SSD as the primary drive, then as with Apple-configured Fusion Drives you will get fast data access when accessing frequently-used files.
This feature is the basis of Apple’s new Fusion Drive technology ; its presence in OS X 10.7 and later allows you to create custom Fusion Drive setups and implement them on Mac systems that did not ship with a Fusion Drive configuration. However, while Apple’s Disk Utility can be used to repair problems with existing Fusion drives, adequately managing the CoreStorage volume currently requires the use of Terminal commands. As a result, for now if you would like to set up a custom spanned-volume in OS X, including using an SSD in a homemade Fusion Drive, you will have to use the following steps: