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Old Jan 26, 2013, 03:11 AM   #1
TheBSDGuy
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Join Date: Jan 2012
How does a Fusion drive appear to the system

A lot of hardware RAID implementations appear under Disk Utility as nothing more than regular hard drives. If you use the command line version of Disk Utility using the command line version "diskutil" it will often do as follows:

$diskutil list

/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *160.0 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_HFS Mountain Lion 75.9 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *320.1 GB disk1
1: EFI 209.7 MB disk1s1
2: Apple_HFS Snow Leopard 320.0 GB disk1s2

(that's a fake list I made up, FWIW but in the above disk1 could be the way a lot of hardware RAID units appear to the system - just as a normal drive even though the RAID unit may have 4 drives in it.

There's a lot of post on the web with titles like "build your own Fusion drive." It uses CoreStorage to set up some drives/volumes into a combined unit.

I'm curious. If someone does a "diskutil list" on a REAL fusion drive, will all the CoreStorage elements show up, or does it, like a RAID unit, simply appear to the OS as just a regular drive.

FWIW, if any of you guys have tried spanning drives with CoreStorage, "diskutil list" will output an entry for each CoreStorage entry and then a final one for the complete, logical volume. In other words, if you used CoreStorage to span 3 disks and make it appear like one big drive, you end up with 3 CoreStorage components and a final usable volume, thus it looks like you have 4 drives on your system.

I'm just curious, because if a Fusion drive appears as a single drive under disk utility, it implies that there's hardware doing a little more than what the guys promoting the "build your own Fusion drive" are saying....or so I think. On the other hand, if each drive inside a real Fusion drive show up as individual drives and volumes under diskutil, then it would kind of imply the actual Fusion drive is nothing more than a glorified SATA extender with an SSD and hard drive installed in it.

This is nothing more than a curiosity question, by the way.

Thanks
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