Ars Technica has detailed (via Tumblr blog posts by Jollyjinx) how Apple’s new Fusion Drive works, an innovative new storage option that gives the performance of flash storage and the capacity of a standard hard drive, unveiled by Apple at iPad Mini event alongside the new 2012 iMac launch.
New Fusion Drive-equipped Macs will come with a 128GB SSD and a much larger hard disk ranging from 1 to 3 terabytes. It was initially explained by AnandTech that the “Fusion” part comes in courtesy of Apple’s software that takes the two independent drives and presents them to the user as a single volume. However, with Jollyjinx successful in building his own Fusion Drive using a 120GB OCZ Vertex 2 connected to his Mac’s SATA bus and a USB-attached 750GB hard disk drive, a lot of questions regarding the new technology have been answered.
Core Storage, is used as the logical volume manager to tie the two physical devices together into a single volume group. Once the volume group is created, Jollyjinx creates a usable HFS+ volume inside of it. This is all accomplished using
diskutil, the command line version of Disk Utility, since the graphical version doesn’t yet support the necessary commands.
Surprisingly, no additional configuration was necessary for the volume to begin exhibiting Fusion Drive-like tendencies. Jollyjinx created 140GB of dummy files and directories on the volume using the
ddcommand, and the system automatically placed about 120GB of those on the SSD before dropping the rest onto the HDD.
Based on these findings, Fusion Drive is indeed a base operating system feature. It detects the SSD part of a drive based on SMART info read across the SATA bus. If a Core Storage volume contains an HDD and an SSD, Fusion Drive appears to be automatically activated.