Using 4k display as second display with 5k iMac

Discussion in 'iMac' started by scottomfg, Oct 21, 2014.

  1. scottomfg macrumors member

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    Oct 21, 2014
    #1
    So, I know it's possible to use a 4k display as a second display with the new 5k iMac, and you can get 3840 x 2160 @60hz using DisplayPort 1.2 with MST. My question, is, does that display use bandwidth on the sole Thunderbolt 2 bus? I plan on running the OWC ThunderBay 4 with two 6g SSDs in RAID 0 as my boot drive (along with some other drives) via TB2, and I'm worried that with a 4k display on the same bus as the RAIDed SSDs that there will be bandwidth issues that will slow things down.

    I've googled a bunch and couldn't find the answer, does a display use the Thunderbolt bus? Or does DisplayPort just connect directly to the GPU?

    Also, two 6g SSDs RAID 0'd via TB2 will be faster (or as fast) for all tasks than the internal SSD that comes in an iMac, right? I plan on getting the 3gb Fusion drive and using it to handle all of my documents and media—using a Fusion drive for those files and a pure RAID 0 dual SSD set-up for my boot drive best fits how I use my machine for work.

    I'm coming from a 6 core Mac Pro, so I haven't had to ask these questions before. Thanks in advance for any help y'all are able to provide.
     
  2. LERsince1991 macrumors 65816

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    #2
    Why do you want to use an external array for a boot drive and the internal for just storage? Seems a little backwards.

    I would guess that dual SSD drives ran via thinderbolt and external case would not be any faster than the internal ssd apple offers. You certainly wouldn't notice an improvement in my opinion.

    I'd go for internal drive as SSD and external drives for storage.

    Not sure about the display question but doesn't the iMac have more than one thunderbolt 2 connection? You could use one for a display and one for hard drives without daisy chaining them?
     
  3. scottomfg thread starter macrumors member

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    #3
    Two words: Flexibility and control. Whatever internal drive I order, effectively I will never have the ability to change it once ordered (I'm not tearing apart an iMac), and I will never have total control over how fast it is (I get whatever drive Apple gives me and that's it). Given these two factors, it seems like a bad way to invest money. Honestly, if I could order the iMac with no internal drive, that's what I'd do. But I don't get that option, so I'm choosing the one that gives me the most utility and requires the least monetary investment.

    Using a 3TB Fusion drive for my internal storage is perfect for my usage: I have just over 1.5TB of photos, InDesign, Photoshop, Illustrator, and audio files I use for my creative work, but in any given month I'm only actively using about 1% of them, which does often include using files that are many years old. So, perfectly tailored to Fusion, which should keep what I'm actively working on in the SSD portion of the drive.

    I want my boot drive to be pure SSD because I need it to always be the fastest I can reasonably make it. I already have a 6g 240 SSD, so I'll buy another for $140, stripe them in RAID 0, and have a super fast .5TB SSD boot drive. In addition to the above creative work, I also do a lot of coding and keep all those files in Dropbox, on the boot volume.

    (The other two drives in my TB2 array will be a 4TB for TimeMachine and a 4TB for all my video files, this machine also will act as home media server)

    I wasn't actually soliciting guesses, but actual knowledge. Many tests have shown RAID 0 external SSDs to be faster than Apple's internal SSDs, but I haven't seen anything that exactly matches my configuration (largely because the 5k iMac is so new), so I was soliciting some expert opinions.

    Judging from the previous gen iMac, the internal SSD connected via PCIe gives about 700 W/R, which is incredible. But two 6g SSDs in RAID 0 should give at least around 775 W and 875 R (extrapolating from here), if not more. Just matching internal performance is enough for me though, as it's cheaper and gives me much more control and flexibility in the future—for instance, I'd always have the option of adding a third SSD drive to the RAID later and getting around 1,000 W/R.

    It does indeed have two ports, and I would indeed use them both, not daisy chain. But both ports share the same bus, and this is where my knowledge becomes insufficient. I'm not sure what the capacity of the bus is, and if the display usage counts against it, or if it even matters. I was hoping someone who knows more than me about this can answer my question. On the new Mac Pros, the 6 TB2 ports are spread over 3 busses, so it is much less of an issue, but on the iMac, all we'll ever have is that one bus to use with TB2/DP.
     
  4. scottomfg thread starter macrumors member

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    #4
    Barefeats just posted some info about the SSD in the 5k iMac they received:

     
  5. paulrbeers macrumors 68040

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    #5
    What are you actually planning on doing with this iMac? Anything in the prosumer/consumer realm would be hard pressed to need anything more than 500MB/s. Unless you are editing RAW 4K video files, I am going to guess that you will see little to no benefit between the internal SSD and an external RAID 0 even if it tops out at 700MB/s internal.
     
  6. scottomfg thread starter macrumors member

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    #6
    Mostly graphic design, but also some video and rendering. I agree, the difference won't be truly felt in the vast majority of my use. However, as I said, using the external drive actually costs less money in my situation, so even if it is just as fast, if not faster, then clearly it is the correct choice for me.

    Also, I really like to think about this stuff a ton when I am making the purchases, and then never again, knowing that I did the best I could at the time. I'm just trying to make sure I get the most capable set-up I can while spending a reasonable amount of money. No reason to buy a slower set-up because I won't use the speed most of the time if the cost is similar, right? If I'm spending $x, I want to make sure I get the most for the $.
     
  7. paulrbeers macrumors 68040

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    #7
    Food for thought:. An external enclOsure is another failure point. Not something I Want in a OS drive....
     
  8. scottomfg thread starter macrumors member

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    #8
    True, it is a failure point, but one not permanently attached to my $3k computer. If it fails, it is very very easy to replace. Likewise, if the computer fails, it doesn't affect my external enclosure. I've used drives in external enclosures for about 15 years now, never had a single enclosure fail, not even once (Every one has been from OWC, probably not a coincidence they have all been super reliable). And it's true that using RAID 0 increases likelihood of drive failure, but I backup very well and regularly, so drive failure is nothing more than a few hours work to restore in the worst case, with essentially zero data loss.
     
  9. Michaelgtrusa, Oct 22, 2014
    Last edited: Oct 22, 2014

    Michaelgtrusa macrumors 604

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    #9
    Where did you here it can drive a 4k display?
     
  10. scottomfg thread starter macrumors member

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    #10
    I didn't. I said it can drive an external 4k display in addition to the internal 5k display.
     
  11. Michaelgtrusa macrumors 604

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    Oct 13, 2008
    #11
    4k is what I meant I did find articles stating this.
     
  12. scottomfg thread starter macrumors member

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    #12
  13. EnderTW macrumors 6502

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    #13
    scott to answer your question. I strongly feel that you'll be fine if you're not daisy chaining. Haswell has 16 pcie lanes. Tb2 takes up 4. Two tb2 is 8. 4 to gpu and 4 to usb Etc.

    You'll be fine.
     
  14. scottomfg thread starter macrumors member

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    #14
    Awesome! That's the info I was looking for! Thank you so much!
     
  15. paulrbeers, Oct 22, 2014
    Last edited: Oct 22, 2014

    paulrbeers macrumors 68040

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    #15

    I'm questioning your logic here.... It's only 1 TB controller that can do two ports. Fairly certain it ISN'T using 8 PCIE lanes.
     
  16. EnderTW macrumors 6502

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    #16
    You're right.

    Sorry, it's one bus, 4X PCIE.

    However, TB2 is cable of 20gb/s in/out. I'm sure you'll be fine.
     
  17. scottomfg thread starter macrumors member

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    #17
    So, the real question here is what kind of bandwidth 4 PCIe lanes provide, right? Anyone have some numbers? EnderTW, believe me, I want you to be right, but given that I plan to use this as my main work machine for years to come, I need to be certain before I commit.

    Any info would be greatly appreciated.
     
  18. xmichaelp macrumors 68000

    xmichaelp

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    #18
  19. theSeb macrumors 604

    theSeb

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    #19
    The question is can Thunderbolt 2 provide enough bandwidth for the 4K set and for your enclosure. DP1.2 is carried through the TB2 and does not have its own pipeline, so it will be taking away from the 20 Gbits/s TB2 pipeline.

    Speculation time:
    DisplayPort 1.2's effective bandwidth is 17.2 Gbits/s. I believe that a 4K monitor running at 60 Hz will use up about 16 Gbits/s. So you have 4 Gbit/s left, which is 476 MB/s. But let's say it uses 17.2, so you have 2.8 Gbit/s left, which is ~ 333 MB/s

    I haven't got any exact numbers of how this will work in practice and I cannot find any. The closest I can find is this from a review of the Pegasus2.


    http://appleinsider.com/articles/14...aid-array-for-apples-latest-professional-macs

    ----------

    No, it is 1000 MB/s. mbps are not MB/s. The 1 GB PCIe Flash based drive can hit the numbers quoted in the article.
     
  20. macmee Suspended

    macmee

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    #20
    Apple says it will run it.

    Apple doesn't say it will run well.

    Running a 4k off the newest rMBP with the quad core i7 and NVIDIA GeForce GT 750M 2048 MB works at 60Hz, made even better by Yosemite. However there is still noticeable mouse lag for me on the 4k screen. If I go into a google hangout or watch a youtube video at the same time as I'm running the 4k, then the lag is unbearable.

    If you plan on hooking up a 4k monitor in addition to the 5k internal display, I strongly advise you to purchase the upgraded gfx card version of this iMac.
     
  21. Cape Dave macrumors 68000

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    #21
    So, perfectly tailored to Fusion, which should keep what I'm actively working on in the SSD portion of the drive.

    The keyword there is SHOULD.
     
  22. EnderTW macrumors 6502

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    #22
    I'll be testing this tonight, picking it up at local store going to return my online order.
     
  23. scottomfg thread starter macrumors member

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    Oct 21, 2014
    #23
    OK, so I have a new theory, informed mostly by this article, including this image:

    [​IMG]

    I don't think the data sent via the Display Port connection goes through the thunderbolt bus, it comes directly from the GPU, which has it's own PCIe connection to the board. So, the two TB2 ports share one thunderbolt bus, but they also share one connection to the GPU.

    So there appears to be three different bottlenecks in play here. 1) The actual cable and chipset in the cable, and each port, which can handle 20Gbps, 2) The Thunderbolt 2 bus itself which also tops out at 20Gbpss, and 3) the DisplayPort 1.2 connection to the GPU which can pump out enough bandwidth to do 4k at 60hz (by pretending it's two displays using MST).

    Soooooo, with this theory the TB2 connection using Port A going to the drive array would have the entire TB2 bus to use, and the Display Port 1.2 connection to the display using Port B would also have the full bandwidth from the GPU available to it.

    This partly explains why if you use a 4k display as your second monitor, you can have no third display—you are using the max output the GPU is capable of all on one Display Port 1.2 connection.

    ----------

    Awesome, I'll be excited to hear. What 4k display are you using?
     
  24. LERsince1991 macrumors 65816

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    UK
    #24
    I tested the retina iMac in store today and drilled it with one of my research data analysis work. The findings?

    MAKE SURE YOU GET MORE THAN THE BASE MODEL.

    The base model seriously lagged, the store had an i5 and 8gb ram. If I were in the market for the iMac i would certainly upgrade everything or the screen will impact the performance way too much. Running a 4k display? Max it out!

    Minimum would be the top graphics card and at least 32gb ram. Shame they charge so much for this...
     
  25. scottomfg thread starter macrumors member

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    Oct 21, 2014
    #25
    Oh yeah def. My config will be 32gb RAM, High-end GPU and i7.

    EnderTW -- you get your iMac?
     

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