Mac Fusion Drive: pro users beware

The Mac Fusion Drive, melding SSD and hard drive, sounded like a great idea. But upon learning more this is another flashy Mac feature that won't work well for pro users, due to OS X's aging architecture.
Written by Robin Harris, Contributor

My initial assessment of Apple's new Fusion Drive was positive, largely because it is the 1st hybrid drive with enough flash capacity to give users an SSD experience - but with much more capacity and at a much lower cost per GB.

But as more details emerged, I've changed my view. Fusion Drive will be good for casual users whose work does not involve processing dozens of GBs of data regularly.

But professionals in biotech, video, music, geophysical and other large data apps where availability is critical should avoid the Fusion Drive. Why?

Fusion confusion
FD is a combination of a hard drive with a 128GB SSD. The glue that holds them together is a special Mac driver that manages the movement of files between the SSD and the hard drive.

Apple's tech note says:
Fusion Drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the hard disk. As a result you'll enjoy shorter startup times, and as the system learns how you work you'll see faster application launches and quicker file access. Fusion Drive manages all this automatically in the background.

This model assumes a user who doesn't process a lot of files and whose files are small. Otherwise the bandwidth required to move files would choke the disk and the SSD capacity would be insufficient.

Users who process multi-gigabyte files regularly - musicians, video editors, scientists, artists - will find irritating starts and stops as the system is forced to move big chunks of data to accommodate new files. There is a better way.

How Seagate does hybrid
Seagate's Momentus XT hybrid algorithms take a different tack: they look for the most frequently used small files and keep them in the flash read cache. This works well because disks are good at retrieving large files, and choke on lots of small files.

Seagate marketing has been too conservative sizing the flash cache, so the SSD benefits aren't consistent for the target market. They are noticably faster, but too often revert back to standard disk performance for busy power users.

Data integrity

Fusion Drive is like Time Machine: a glitzy feature that power users turn off because it hurts performance. 

But there's a better reason for power users to avoid the Fusion Drive: data integrity. 

HFS+ is primitive 1980s file system tech. There's no background data scrubbing, checksums, or other modern data integrity features.

HFS+ assumes that writes gets written as intended and that whatever is read is what it intended - but due to many system-level issues that isn't a safe assumption. With FD your valuable files will get read and written a lot more - and face corruption every time.

The Storage Bits take
People who make a living on their Macs working with big files shouldn't risk letting HFS+ move their data around any more than they must. There will be corruption and you won't know it until it is too late. That makes FD a bad idea for pros.

Casual users will like it: it will make their Mac snappier and hide the creakiness of the aging Mach kernel. Consumers who don't have a lot of data - and most have no idea - will find themselves operating out of the SSD almost all the time and will love it.

And if they lose a file or 2? Hey, stuff happens. How often will they notice?

Comments welcome, of course. Lesson learned: act in haste, repent at leisure.


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