iMac & DIY Fusion Drive - slow speeds

Discussion in 'iMac' started by tennisguy, Apr 5, 2014.

  1. tennisguy, Apr 5, 2014
    Last edited: Apr 5, 2014

    tennisguy macrumors newbie

    Apr 5, 2014
    Hi all. I have a 3.2GHz i5 w 24 GB of Ram. Purchased in January 2014.

    My good friend helped me install a DIY Fusion. I purchased a 256GB Samsung 840 Pro SSD and he did the work to "fuse" it with the regular 1TB HD the iMac came with. It is attached via USB 3.0 in an OWC case.

    Originally, after being set up - Black Magic Speed tests benched it at 250-350 write and 500-650 read. I'm not a techie, but spent enough time to know that is pretty smoking and well above a typical disk hard drive.

    Well, now it is down to 70-80 write and 180-250 read. The imac still boots in less than 25 seconds, so I don't feel the system is slow. (and it certainly blazes next to the first generation aluminum iMac I had before - a 2008 model). But I am concerned at deterioration of speed and really thought the extra investment would have led to faster sustained speeds.

    So my questions are:
    * Does anybody have any ideas why this is happening?
    * Any idea what the fix is? I'm hoping not to have to start over.
    * Any ideas whether it would have been better just to keep the 256 a boot drive for OS & apps, and leave the 1TB for media? I almost did this and wonder if it would have simplified things or led to better performance.
    * Any idea where to go from here?

    BTW, I know a little about TRIM, but I think my friend said the Samsung will handle that itself.

    Lastly, you can probably tell I know a bit ABOUT this stuff, but not much on HOW to do it. So regular maintenance and upkeep is not my strong suit.

    Thanks in advance for comments & suggestions.

  2. Blue Sun macrumors 6502a

    Feb 11, 2009
    The Samsung drive does have garbage collection functionality built-in, but the drive will sustain better performance if TRIM is active. Unfortunately you cannot activate TRIM over USB3, but it is supported over Thunderbolt.

    I'm not sure why the performance has dropped according to Black Magic speed test. If I were to guess I'd say that the SSD is full and therefore the test is running purely on the mechanical drive, which explains the slow write speeds.

    You mentioned that the machine is still fast, so just keep using the system as normal - don't put too much emphasis on the Black Magic benchmark.

    But if it were me, I would keep the SSD and traditional HDD separate and use the SSD purely as a boot drive with the OS and apps. That way, you decide what lives on the SSD, not some Apple algorithm.
  3. Fishrrman macrumors G5


    Feb 20, 2009
    I will second blue sun's reply above as being the way to go.

    I realize it will involve some work, but my suggestion is to backup the existing "fused" drive, then "separate" the fused drives into two "separate" drives (SSD and HDD), then put your OS and apps on the SSD, and move space-clogging stuff like movies, pics, and music to the HDD.

    It's really not difficult to manage two drive icons on the desktop, instead of one. I keep no less than SEVEN mounted volumes on my desktop all the time, and find it relatively easy to find things when I need to....
  4. tennisguy thread starter macrumors newbie

    Apr 5, 2014

    Thanks to both of you for your thoughts. Do you recommend separating the SSD boot drive & internal spinning drive for reasons of:
    1. Control - so you can be in charge of what data is held on what drive?
    2. Speed - to keep a purity behind the SSD capabilities and actual function?
    3. A little big of both?

    Right now, a restart will re-launch in 25 seconds and a fresh boot (from shutdown) launches in 45 seconds. I'm not blown away at snappiness, but the system certainly isn't slow. But it's only three months old. My main concern is I'm simply not sure I'm getting the speed advantage I anticipated when buying the Samsung SSD.

    Perhaps the DIY Fusion is working fine and Black Magic just can't capture it. Or maybe its lagging and has the wrong info on the wrong drives and isn't self correcting. I really don't know any other way to troubleshoot it.

  5. Blue Sun macrumors 6502a

    Feb 11, 2009
    I'd say a little bit of both. Even Apple admits that the FD isn't as fast as Flash storage. The 45 second startup isn't great IMO, but I'm sure using the Samsung SSD as an independent volume will speed things up.

    FWIW, the Black Magic benchmark probably isn't optimised for the FD setup, but nevertheless, that boot up time will probably improve without the FD.
  6. marzer, Apr 7, 2014
    Last edited: Apr 7, 2014

    marzer macrumors 65816


    Nov 14, 2009
    I run a couple of computers (iMac and Mac mini) with DIY Fusion drives. Something to keep in mind is the Fusion drive reserves 4GB buffer on the SSD for HDD operations. The default test file size on Blackmagic is 5GB.

    With that said, before redoing your drive, I'd recommend: install TRIM Enabler to enable TRIM on the SSD. This helps with long term SSD operation. Good practice even if it doesn't have immediately noticeable results on your SSD performance. Folks who argue against TRIM simply don't understand its relation to garbage collection. TRIM and garbage collection are NOT the same thing, TRIM compliments the built-in garbage collection process to make it more efficient over time.

    Also reduce the Blackmagic test file size to 4GB or lower. Both my Fusion Drive systems have more data than the SSD can hold by itself, and with a smaller test file both give SSD speed results. As I rarely manipulate large data files, most of my user activity pretty much runs at SSD speeds.

    Now if your important computer uses involve frequently manipulating data sets larger than 4GB, you may want to consider splitting it up. If you enjoy the convenience of Fusion Drive, as I do, give my suggestions a try before going through the hassle of changing your config.
  7. macthefork macrumors 6502

    Feb 2, 2013
    I would consider it fine to run a SSD by itself on USB 3. However, running a Logical Volume (Fusion Drive) that includes a USB 3 device is probably asking for trouble. There have been many reports of USB 3 drives unmounting, especially after sleep, for various reasons such as the USB controller not being fast enough to "see" the drive immediately upon wake. If the SSD doesn't mount when expected, it will likely corrupt your Logical Volume.

    It can work if an upgraded (newer) USB controller is in use. But, as mentioned, USB doesn't support TRIM. TRIM is not the same as the Garbage Collection that all SSD controllers provide. This link does a pretty good job of describing the differences between TRIM and Garbage Collection.

    Thunderbolt so far has proven to be more stable than USB 3 to date, and as mentioned TRIM is supported through Thunderbolt. For a 256 GB drive, something like this, or a Thunderbolt enclosure would be sufficient and more reliable for a Logical Volume.

    I currently use a 256 SSD connected to Thunderbolt to test different Operating systems. (I have a 480GB installed internally that I use for the Mac OS). I've never had any problems with the Thunderbolt interface dropping.
  8. MacLabTech macrumors newbie

    Apr 7, 2014
    North Carolina
    Since it is an External Fusion do the fans run at a high speed?

    I'm suppressed a fusion would work externally since the logic board firmware would be looking for a temperature sensor for the SSD.
  9. tennisguy thread starter macrumors newbie

    Apr 5, 2014
    Good thoughts to everyone contributing. Thank you!

    My SSD (Samsung 840 Pro 256 gb) is currently in an OWC on the go pro USB enclosure. So while I have downloaded Trim Enabler, I am fairly certain it won't work over USB 3.

    So my first question is - if I buy the Seagate TB sled or the Delock TB enclosure, can i just turn off the cpu, move the SSD into the new TB connected unit and fire up the cpu again? Or does that disconnect what Fusion had created?

    I am leaning towards trying that (if it will work) before giving up on Fusion. If that does not work (or cannot be switched from USB to TB that way), then I will probably get a TB enclosure anyways and set up from scratch (sigh) with a boot disk.

    Any other thoughts on that?

  10. macthefork macrumors 6502

    Feb 2, 2013
    You shouldn't have any problems switching from a USB to a Thunderbolt drive for the Logical Volume. During boot, the Mac will initialize (and identify) the hardware prior to looking for and beginning the loading of the OS. This process should recognize the new Thunderbolt SSD and the internal Drive as the Fusion drive components, then the boot loader starts loading the kernel...

    Before making the switch, though, it's always a good idea to clone or backup your volume so if there is any problem, you can simply create the Thunderbolt/internal Fusion drive volume again, then clone or replace everything back onto it easily.
  11. tennisguy thread starter macrumors newbie

    Apr 5, 2014

    All, I just got a DeLock Thunderbolt enclosure and moved the SSD into it. Booted up and start up was a bit less than 15 seconds. The whole CPU is significantly snappier. I benchmarked it, and it scored about the same, but I'm assuming the Fusion drives don't bench well for some reason - because I clearly got a speed gain.

    Also, while I had backed up the Fusion drive first, switching enclosures had only one noticeable glitch in going from USB to TB. My Microsoft office key deactivated, which means a one hour call with their inept tech support. :(

    But all in all, good outcome. I'll stick with Fusion for now, but if I ever have to start over, will do boot drive only via TB. Thanks for your thoughts/input! :)
  12. macthefork macrumors 6502

    Feb 2, 2013
    Be sure to enable Trim... that also should help speed things up, especially over time.
  13. tennisguy thread starter macrumors newbie

    Apr 5, 2014
    Yes, TRIM is enabled. A restart last night was about 6 seconds from sound to usable desktop. Unreal.

    Only downside so far is the Delock is louder than I thought. I thought the SSD would be near silent, but there is a steady hum. It is about the same as a regular HDD external, so not super bad - but a new sound nonetheless.

    But all in all, would rather have hums AND performance than none at all. :)
  14. SaSaSushi macrumors 68040


    Aug 8, 2007
    Takamatsu, Japan
    Is that the Delock 42490 enclosure? If so, I'm using the same one and thanks to it being fanless it is dead silent even when I hold it against my ear.

    I'm using a 500Gb 840 EVO. Perhaps your 840 Pro is generating the noise or you have a defective enclosure.
  15. tennisguy thread starter macrumors newbie

    Apr 5, 2014

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