Apple's Fusion Drive: An Inside Look

Amid the hubbub over the iPad Mini, Apple last week announced a hybrid flash/hard disk technology for iMacs called Fusion Drive. Automated hybrid systems like the Fusion Drive promise SSD-like performance with hard-disk-like terabytes of capacity--a powerful and attractive combination. Fusion Drive is the first mainstream hybrid for desktops, and I hope mainstream PC makers follow Apple's lead.

In typical Apple fashion, the announcement had lots of dazzle and little technical detail, but a few specifications emerged. Fusion Drive uses separate SSD and HDD to provide 128 Gbytes of flash and 1T to 3Tbytes of spinning disk.

I guessed Fusion Drive's magic happened via caching. It seemed reasonable. Several caching products are already available for Windows PCs. Intel's SRT (Smart Response Technology) is the best-known caching solution for PCs. It uses up to 64 Gbytes of SSD as a cache managed by the RAID features of its Z68 chipset. This approach limits SRT to enthusiasts, such as this humble reporter, who are willing to build a PC using a Z68 motherboard.

Users with more conventional PCs can buy SSDs with bundled caching software from SANdisk, whose ReadyCache SSD comes bundled with Condusive's ExpressCache. (Condusive is best known for its DiskKeeper defragmentation software). OCZ and the Crucial division of Micron bundle Nvelo's Dataplex with its Synapse and Adrenaline caching SSDs.

However, I was wrong. My friend Stephen Foskett's announcement-day blog post, which described the Fusion Drive as file-level tiering, was closer to the mark.

Apple's Fusion Drive marries the kind of sub-LUN tiering that many hybrid disk arrays perform as well as Flash-based write caching. All of this is managed through Apple's core storage logical volume manager. Sub-LUN tiering is more complex than caching because it requires a storage system, or operating system in this case, to track data access frequency for all the data on the disk.

When an SSD and hard drive are combined into a single volume in Apple's core storage, the volume manager will dedicate 4 Gbytes of the SSD as a write cache and place newly written data there. Frequently accessed data will be promoted from the hard disk to the SSD, as well. The 128 Gbytes of flash Apple uses should be enough to hold all the programs and data most desktop users access more than once in a blue moon, giving those programs SSD-like performance.

At the announcement, Apple execs said Fusion Drive would be available only with new iMacs and Mac Minis. However, Tumblr blogger Jolyjinx, whose real name is Patrick Stein, discovered that the secret sauce is all in core storage in the latest version of OSX. He found this out by enabling Fusion Drive on his older Mac with a plain-vanilla OCZ SSD. He even managed to run ZFS on top of Fusion Drive, though I would probably use ZFS's SSD support with mirrored SSDs for the ZIL (write cache).

All in all, Fusion Drive is a big step up for Mac users. I hope some PC vendors will start bundling ExpressCache or Dataplex with their wares and bring similar performance to the masses. Of course, Microsoft could just add caching to Windows, too.