Is it possible to build a DIY Fusion drive using 3 drives – 2 HD's and 1 SSD?
Will the Fusion machinery optimise on three levels i.e. SSD, then fast HD, then slow HD?
I have a mid 2011 27" iMac, with a 1 terabyte internal HD. I also have a spare 1TB HD and a 256GB SSD recovered from another dead Mac. I'm going to put one of these in using the iFixit installation kit, and the other in a caddy in place of the DVD drive. Once I've done that, can I set all three up as a fusion drive?
Best Answer
It is possible to create a Logical Volume Group spanning over more than two disks. I doubt that it will differentiate between fast and slow HDD. The SSD part should work though.
To accomplish this put all drives in your iMac and boot to a Mavericks or Yosemite bootable installer thumb drive.
Partition all drives as simple volumes with Disk Utility.
Then open Terminal and enter
diskutil list
to get an overview. The result is similar to this one:Now create the Logical Volume Group with:
Choose the proper disk identifiers found in the diskutil list.
The output will be similar to this one:
Now create a new Logical Volume with:
with the example above this is
This will yield the following:
The 2 TB is erroneous: in fact a ~2,25 TB volume will be built:
Now install a new system and enjoy your DIY SuperFusion Drive.
One fact cannot be concealed: the SuperFusion Drive is even more vulnerable to drive failure than a normal Fusion drive. The failure probability is comparable to a RAID0.
Here is a comparison for 1, 2 or 3 disks:
Survival probability (for new disks and being optimistic):
Considering that at least one drive is already 5 years old the survival probability is even worse. So better get a reliable backup drive!
A deleted question invisible for some or most users asks for a speed test. A (Super)Fusion Drive is no RAID0/5/6, so r/w speed directly depends on the physical disk written to/read from. Usually at least 4 GB of the SSD are kept free. One can expect that the first ~4 GB written to the DYI SuperFusion Drive are transferred with SSD speed (150-550 MB/s) and the rest with HDD speed (20-120 MB/s). The read speed depends on the location of the files (i.e. SSD or HDD) and is similar to the write speed.