Ars has its Fusion Drive-equipped Mac Mini in hand, and I've begun digging into it. We'll do a full review next week, but it's immediately obvious that many of the suppositions about Fusion Drive were correct: the technology uses Core Storage to bind together the physical hard disk drive and the physical solid state drive into a single volume. The aggregate capacity is the sum of the two, minus system partitions for booting and recovery.

Contrary to rumor, Finder sees but a single drive, as does the graphical version of Disk Utility:

We have to go to the command line to get the rest of the story. Listing all the volumes with diskutil shows the following:

LeeHs-Mac-mini:~ leeh$ diskutil list /dev/disk0 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *121.3 GB disk0 1: EFI 209.7 MB disk0s1 2: Apple_CoreStorage 121.0 GB disk0s2 3: Apple_Boot Boot OS X 134.2 MB disk0s3 /dev/disk1 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *1.0 TB disk1 1: EFI 209.7 MB disk1s1 2: Apple_CoreStorage 999.3 GB disk1s2 3: Apple_Boot Recovery HD 650.0 MB disk1s3 /dev/disk2 #: TYPE NAME SIZE IDENTIFIER 0: Apple_HFS Macintosh HD *1.1 TB disk2

There are three disk devices exposed: disk0 , the SSD; disk1 , the HDD; and disk2 , the Core Storage volume. The two physical disks have several partitions, but the third device has only the single Apple_HFS partition listed.

Requesting additional info on each one gets us some more details:

LeeHs-Mac-mini:~ leeh$ diskutil info disk0 Device Identifier: disk0 Device Node: /dev/disk0 Part of Whole: disk0 Device / Media Name: APPLE SSD SM128E Media Volume Name: Not applicable (no file system) Mounted: Not applicable (no file system) File System: None Content (IOContent): GUID_partition_scheme OS Can Be Installed: No Media Type: Generic Protocol: SATA SMART Status: Verified Total Size: 121.3 GB (121332826112 Bytes) (exactly 236978176 512-Byte-Blocks) Volume Free Space: Not applicable (no file system) Device Block Size: 512 Bytes Read-Only Media: No Read-Only Volume: Not applicable (no file system) Ejectable: No Whole: Yes Internal: Yes Solid State: Yes OS 9 Drivers: No Low Level Format: Not supported Device Location: "Lower"

LeeHs-Mac-mini:~ leeh$ diskutil info disk1 Device Identifier: disk1 Device Node: /dev/disk1 Part of Whole: disk1 Device / Media Name: APPLE HDD HTS541010A9E662 Media Volume Name: Not applicable (no file system) Mounted: Not applicable (no file system) File System: None Content (IOContent): GUID_partition_scheme OS Can Be Installed: No Media Type: Generic Protocol: SATA SMART Status: Verified Total Size: 1.0 TB (1000204886016 Bytes) (exactly 1953525168 512-Byte-Blocks) Volume Free Space: Not applicable (no file system) Device Block Size: 512 Bytes Read-Only Media: No Read-Only Volume: Not applicable (no file system) Ejectable: No Whole: Yes Internal: Yes Solid State: No OS 9 Drivers: No Low Level Format: Not supported Device Location: "Upper"

LeeHs-Mac-mini:~ leeh$ diskutil info disk2 Device Identifier: disk2 Device Node: /dev/disk2 Part of Whole: disk2 Device / Media Name: Macintosh HD Volume Name: Macintosh HD Escaped with Unicode: Macintosh%FF%FE%20%00HD Mounted: Yes Mount Point: / Escaped with Unicode: / File System Personality: Journaled HFS+ Type (Bundle): hfs Name (User Visible): Mac OS Extended (Journaled) Journal: Journal size 90112 KB at offset 0x1357c000 Owners: Enabled Content (IOContent): Apple_HFS OS Can Be Installed: Yes Media Type: Generic Protocol: SATA SMART Status: Not Supported Volume UUID: 268AF169-437C-36DA-B8FE-403E20F6B68D Total Size: 1.1 TB (1115853029376 Bytes) (exactly 2179400448 512-Byte-Blocks) Volume Free Space: 1.1 TB (1101533347840 Bytes) (exactly 2151432320 512-Byte-Blocks) Device Block Size: 512 Bytes Read-Only Media: No Read-Only Volume: No Ejectable: No Whole: Yes Internal: Yes Solid State: Yes OS 9 Drivers: No Low Level Format: Not supported Device Location: "Lower"

The last device, disk2 is the Fusion Drive. Interestingly, it identifies itself as a solid state disk (see near the bottom), which likely affects OS X's treatment of it with respect to hotfile clustering (OS X's sort-of defragmentation scheme).

Running diskutil again and this time requesting info about available Core Storage volumes yields what we've been looking for: Fusion Drive made of Core Storage.

LeeHs-Mac-mini:~ leeh$ diskutil cs list CoreStorage logical volume groups (1 found) | +-- Logical Volume Group 31CBA650-4ABC-4DA6-AA73-6D96B36693E2 ===================================================== Name: Macintosh HD Size: 1120333979648 B (1.1 TB) Free Space: 0 B (0 B) | +-< Physical Volume F4A50F9C-841D-4C6D-AFC4-6A10560E0717 | ---------------------------------------------------- | Index: 0 | Disk: disk0s2 | Status: Online | Size: 120988852224 B (121.0 GB) | +-< Physical Volume C86789FD-1978-453A-BCA7-81DC05AA66C7 | ---------------------------------------------------- | Index: 1 | Disk: disk1s2 | Status: Online | Size: 999345127424 B (999.3 GB) | +-> Logical Volume Family AA92FDF8-F962-4DE3-AC2F-1B6ABF22AA22 ------------------------------------------------------ Encryption Status: Unlocked Encryption Type: None Conversion Status: NoConversion Conversion Direction: -none- Has Encrypted Extents: No Fully Secure: No Passphrase Required: No | +-> Logical Volume B97C9558-14FC-4BFC-B413-AFB292BD29F0 ----------------------------------------------- Disk: disk2 Status: Online Size (Total): 1115853029376 B (1.1 TB) Size (Converted): -none- Revertible: No LV Name: Macintosh HD Volume Name: Macintosh HD Content Hint: Apple_HFS

Specifically, the two physical devices (the HDD and the SSD) are grouped together into a single Logical Volume Group, which in turn contains a Logical Volume Family made up of a single Logical Volume. That Logical Volume, disk2 , is our Fusion Drive.

OS X is definitely aware that it has two disks, though. Even though only disk2 is visible, System Information knows that there's both an SSD and an HDD in there.

Stay tuned for our full hands-on with Fusion Drive and the Mac Mini, coming next week!