Apple's recently introduced Fusion Drive system is available in Apple's current Mac Mini and will be included as an option for other Mac systems starting in November and December of this year. This new drive setup has its benefits, but also some limitations, and to address these Apple has issued a FAQ that addresses troubleshooting and how the drive can be used and configured.
A Fusion Drive is essentially a hybrid volume that spans both a solid-state drive (SSD) and a hard drive installed in the Mac. As large SSD drives are unfortunately still rather expensive, to get the speed benefits of an SSD while still having ample storage, manufacturers have resorted to hybrid approaches. Some of these have been standalone drives augmented with NAND flash chips, such as Seagate's Momentus XT series; however, the amount of flash storage in this type of drive is relatively small, around 4GB to 8GB.
Another approach has been tried by Intel with the SSD Caching feature introduced in its Z68 chipset, where a small SSD of up to 64GB in size could be used to dynamically cache data from a larger hard drive. This chipset was first used in Apple's iMac systems, but this feature was not made available and users could not configure and use SSD Caching.
Instead of using a hardware-based setup, Apple's approach with Fusion Drive involves a software-based volume manager called CoreStorage, which breaks from traditional hard-drive use by providing a management layer between the partitioned device and the OS, giving more versatility in how storage volumes are handled. Instead of directly presenting drives and partitions to the OS, CoreStorage organizes them into logical-volume groups where it can add extra functionality to them underneath the OS.
This management layer allows for special handling of storage volumes, including the full-disk encryption that was introduced with FileVault 2 in Lion, but also supports volume-spanning features where partitions from different storage devices can be combined into one logical volume for the OS to use. Using this feature, Apple has simply combined an SSD with a hard disk into a logical-volume group that is presented to the OS for use.
This approach is similar to Intel's SSD Caching option, except that there are no size limitations and it is not managed in hardware. While for now Apple has started with 128GB SSDs for Fusion Drive on its Mac Mini systems, SSDs of any size can be used in combination with any hard drive to create a hybrid volume.
In addition to there being no size limitations, volume spanning will inherently prioritize the primary device in the CoreStorage logical-volume group, meaning that when the SSD and hard drive are combined, if the SSD is set to be the primary volume then the system will prioritize using it for storing files, so as files are used CoreStorage will move them to this drive. In this way, by using CoreStorage to combine a separate SSD with an HDD you end up with a hybrid and optimized system, which Apple calls Fusion Drive.
While overall beneficial for users, Fusion Drive does have some limitations that may hinder some uses of the drive. The first is that only internal drives are supported for use with Fusion Drive. Apple's iMac and Mac Mini systems have options to include dual drives so they can be configured with Fusion Drive, but other Mac systems (especially laptops) do not have this ability by default. While CoreStorage support in these systems means they can technically use Fusion Drive technology, you would have to first find a way to install dual drives (in a post last year I outlined how to do install a dual drive in a 2009 MacBook Pro).
Even though external drives are not supported, it is likely that you can customize CoreStorage for use with such drives; however, such a setup is not recommended and would increase the chance of data corruption should a drive be inadvertently disconnected.
Another limitation with Fusion Drive is with partition management, as Apple only supports the addition of one extra partition to the hard drive (not the primary SSD in the logical-volume group). It is likely that with customized manual setups you can configure volume spanning with more partitions, but by default Apple only supports the one option.
If you partition the drive, the new partition will not be part of the logical-volume group that Fusion Drive uses, so it will not benefit from the speed of the SSD; however, it will allow you to install Windows using Boot Camp. The use of CoreStorage technology requires OS X, so Windows installations will not be able to use Fusion Drive, but the separate partition that is not managed through CoreStorage will allow Windows to work. This also means that when using Boot Camp you will not be able to access the Mac's data, even if you install support for the HFS+ format in Windows (using tools like MacDrive).
A final limitation is that unlike previous drive setups that could be managed through any version of Disk Utility, managing Fusion Drive requires the more updated version of Disk Utility that is included with OS X 10.8.2 or later. Therefore, if you are experiencing problems with the drive and are attempting troubleshooting in Target Disk mode with another Mac, that Mac should probably be running OS X 10.8.2.
Apple's recently published FAQ on Fusion Drive can be found here.