Fusion Drive & Hard Drive Spin Down

[justreturned]

Dear community,

I have a 2010 iMac with built-in 256 GB SSD drive and a 2 TB hard disk. Currently I am using the SSD as my main boot drive for system and data, and the hard drive for offloading any data I do not currently need. Advantage of this setup is that I can unmount and eject the hard drive, making my iMac virtually noise-free and super responsive.

With the new option to create a fusion drive even on older iMacs such as mine, I find the possibility not to have to manually reshuffle data between my two drives very attractive. However, I am concerned about the ability of the HDD to spin down if unused.

Can anybody with first-hand experience running a fusion drive on such a setup post some details on whether at all the HDD is still allowed to spin down, and in what kind of regularity it happens? Naively I would assume that unless the data on the fusion drive exceeds the capacity of the SSD, the HDD would never actually have to spin up. However I am curious about hearing some insight.

Many thanks for sharing your experience.

Stephan

[sth]

http://arstechnica.com/apple/2012/11...fusion-town/2/

Several folks asked in the Fusion Drive article comment thread about the HDD's behavior when drive spin-down is enabled in the system's power saving options—specifically, if IO activity directed to the SSD will cause the HDD to wake and spin up if it's asleep. To test, I forced the HDD to spin down after one minute of inactivity with sudo pmset -a disksleep 1, then started up iostat to keep an eye on each physical drive's activity. The system at that point was freshly reloaded with OS X, so I copied a large file to my profile's Downloads directory, hit Command-C to queue the file up in the clipboard, and then waited for the HDD to spin down. This happened on schedule, about a minute later, and was clearly audible; once the drive had spun down I hit Command-V to paste a copy of the file into the directory. Immediately, the HDD spun up, though iostat showed no activity on the HDD at all.

I was able to repeat this several times, and the result was the same each time—any IO directed to the Fusion Drive volume while the HDD was spun down caused the drive to spin back up, even if the HDD wasn't used to service any of the IO. The testing methodology isn't super-scientific, but the observed behavior was at least consistent, so users who want to eke out those extra pennies per month from having their HDDs spun down, beware.