Using 4k display as second display with 5k iMac

[theSeb]

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.

Each port does not provide 20 Gbps. There is only one Thunderbolt bus in the iMac with 2 ports (historically and I am pretty sure it's the same this time).

It is 20 Gbps in total for the bus. The display port signal will consume its own chunk out of that 20 Gbps and whatever is left will be the bandwidth for your external hard drive enclosure. Your sequential reads and writes will be affected a bit.

 

[scottomfg]

Each port does not provide 20 Gbps. There is only one Thunderbolt bus in the iMac with 2 ports (historically and I am pretty sure it's the same this time).

It is 20 Gbps in total for the bus. The display port signal will consume its own chunk out of that 20 Gbps and whatever is left will be the bandwidth for your external hard drive enclosure. Your sequential reads and writes will be affected a bit.

I'm not sure you read what I wrote: I said very specifically already that there is only one thunderbolt bus for the two ports. The question is does the display port connect directly through the GPU to the Thunderbolt/Display Port, or does it go through the thunderbolt bus? That is the only question we're trying to answer here.

If you still think your answer is in response to this question, do you have any source for your info? Without a source, it's just another theory, like this whole thread. Thanks in advance.

 

[EnderTW]

Actually had to wait until today. I'll be grabbing it in a couple hours.

 

[theSeb]

I'm not sure you read what I wrote: I said very specifically already that there is only one thunderbolt bus for the two ports. The question is does the display port connect directly through the GPU to the Thunderbolt/Display Port, or does it go through the thunderbolt bus? That is the only question we're trying to answer here.

If you still think your answer is in response to this question, do you have any source for your info? Without a source, it's just another theory, like this whole thread. Thanks in advance.

It all goes through the bus (controller).

 

[scottomfg]

It all goes through the bus (controller).

Image

Oh man, if that's the case, it's a real bummer. Thanks for the graphic, that seems pretty conclusive.

So, if you can't get any decent use of an array and a 4k display simultaneously, perhaps using USB3 creatively is an option.

Do we know how many PCIe lanes are dedicated to the 4 USB 3 ports?

Thanks again!

 

[theSeb]

Oh man, if that's the case, it's a real bummer. Thanks for the graphic, that seems pretty conclusive.



Thanks again!

I have to say though that you have asked some interesting questions and this thread has made me question my understanding of Thunderbolt, so I have been rereading a lot and found this little gem:

Intel claims Thunderbolt 2 will be able to transfer a 4K video while simultaneously displaying it on a discrete monitor.

I am not sure what this claim means exactly and "will be able to transfer a 4K video" is a very subjective term to make, because it does not specify how quickly that transfer will happen. I mean USB 2 could transfer a 4K video. It will take a long time, especially if the video is long, but a 2 second 4K video will not take that long via USB 2.

I would still love to see a real test of this, because all I have found is speculation, even though theoretically, it makes sense.

So, if you can't get any decent use of an array and a 4k display simultaneously, perhaps using USB3 creatively is an option.

Do we know how many PCIe lanes are dedicated to the 4 USB 3 ports?

USB 3 with UASP will limit the full potential of a "fast" modern SSD, when it comes to sequential reads and writes, which I assume are important to you, since you're planning on running two SSDs in RAID 0.

http://electronicdesign.com/embedded/whats-difference-between-usb-uasp-and-bot?page=2

 

[scottomfg]

At Mac Performance Guide, Lloyd did some interesting tests with USB speed on a nMP and a rMBP, http://macperformanceguide.com/MacPro2013-USB3-performance-limitations.html.

What I'm interested in is Striping two drives in separate enclosures on separate USB3 ports, hopefully on separate PCIe lanes, as he did on the rMBP. Hopefully the 5k iMac has this potential.

 

[theSeb]

At Mac Performance Guide, Lloyd did some interesting tests with USB speed on a nMP and a rMBP, http://macperformanceguide.com/MacPro2013-USB3-performance-limitations.html.

What I'm interested in is Striping two drives in separate enclosures on separate USB3 ports, hopefully on separate PCIe lanes, as he did on the rMBP. Hopefully the 5k iMac has this potential.

Sounds like a good plan.

 

[EnderTW]

Hi Scott,

Man.. this 5K display is out of the world.

The picture doesn't do it justice, the 4k Dell is out of focus, but it runs great. Verge is smooth as butter, nothing like the first gen retina MBP.

Sorry can't test TB2, don't have an enclosure, but rest assured your 4k + 5k display will be working great, and believe me, you can tell a difference from 4k to 5k.

 

[WilliamG]

Each port does not provide 20 Gbps. There is only one Thunderbolt bus in the iMac with 2 ports (historically and I am pretty sure it's the same this time).

It is 20 Gbps in total for the bus. The display port signal will consume its own chunk out of that 20 Gbps and whatever is left will be the bandwidth for your external hard drive enclosure. Your sequential reads and writes will be affected a bit.

Apple seems to imply that both ports do 20Gbps each...

http://www.apple.com/imac-with-retina/performance/

 

[theSeb]

Apple seems to imply that both ports do 20Gbps each...

http://www.apple.com/imac-with-retina/performance/

That little animation seems to suggest that each port is on a separate bus and they found extra PCIe lanes from somewhere. I find it hard to believe since the design has traditionally been like this:

Only four lanes are used by Intel's Thunderbolt controller, the remaining lanes are used for things like Bluetooth and WiFi. Do the math and you'll realize that four PCIe 2.0 lanes are only good for 20Gbps of bandwidth, plus DMI between the Z68 chipset and Sandy Bridge is limited to 20Gbps itself. A single Thunderbolt port is capable of 20Gbps of bandwidth (10Gbps in each direction), so that works out well (if you don't use any of the other PCIe devices in the system at the same time). While the 21.5-inch iMac has a single Thunderbolt port, the 27-inch model has two. That's a total of up to 40Gbps of bandwidth to Thunderbolt devices, but only 20Gbps to the controller itself. Don't be fooled by the presence of two Thunderbolt ports on the 27-inch iMac, you don't get any more bandwidth than you would on the 21.5-inch model

What we need is someone to look into the system report on the riMac and take a screenshot of the thunderbolt bit. Then we will know for sure, if there is one bus or two.

Here is what 3 busses look like

 

[Gav Mack]

You'll find users comments on the Pegasus 2 and others over in the Mac pro section.

I will say this about booting OS X primarily from thunderbolt - a lot have ended in tears it's risky and I certainly wouldn't recommend it. It looks good on paper, but in practice..

Two I know have moved their media and used aliases onto a slower array like the drobo 5d stocked with WD Blacks, the optional accelerator card which is pretty good and reverted to booting off their internal drive.

 

[theSeb]

You'll find users comments on the Pegasus 2 and others over in the Mac pro section.

I will say this about booting OS X primarily from thunderbolt - a lot have ended in tears it's risky and I certainly wouldn't recommend it. It looks good on paper, but in practice..

Two I know have moved their media and used aliases onto a slower array like the drobo 5d stocked with WD Blacks, the optional accelerator card which is pretty good and reverted to booting off their internal drive.

Good points. I actually completely missed the bit about the intention to boot off the Thunderbolt enclosure.

I assumed it would be used for work files.

 

[WilliamG]

You'll find users comments on the Pegasus 2 and others over in the Mac pro section.

I will say this about booting OS X primarily from thunderbolt - a lot have ended in tears it's risky and I certainly wouldn't recommend it. It looks good on paper, but in practice..

Two I know have moved their media and used aliases onto a slower array like the drobo 5d stocked with WD Blacks, the optional accelerator card which is pretty good and reverted to booting off their internal drive.

I've been booting off Thunderbolt for a hair under two years now with never a single issue on my 2012 iMac. I'm just using a Seagate Backup Plus Thunderbolt enclosure with a 256GB Samsung 830 SSD, mind,

 

[theSeb]

I've been booting off Thunderbolt for a hair under two years now with never a single issue on my 2012 iMac. I'm just using a Seagate Backup Plus Thunderbolt enclosure with a 256GB Samsung 830 SSD, mind,

I have heard tales of bumps disconnecting the cable accidently. I have never had that happen with TB devices, but mine are not portable and sit hidden away.

 

[WilliamG]

I have heard tales of bumps disconnecting the cable accidently. I have never had that happen with TB devices, but mine are not portable and sit hidden away.

I'm not sure there's much difference between "bumping" a cable and just pulling the plug on the iMac. Either way, I can't see how it would happen if you're even remotely careful, or, like I do - use a TwelveSouth Backpack!

FYI, I did test this back on a fresh install. I pulled the Thunderbolt cable out of the computer twice in a row, and it did no harm. The OS froze, I turned off the computer, plugged it back in, and booted back up.

 

[Gav Mack]

Good points. I actually completely missed the bit about the intention to boot off the Thunderbolt enclosure.

I assumed it would be used for work files.

That was the point where my eyes went wide open and I thought of Harry Enfield "nooo, you don't want to do that".

If a client demanded he wanted me to setup his 5k booting OS X over tb2 in raid 0 I would have to shake his head vigorously till he changed his mind. A single drive is ok, a big array would be even better and faster but raid 0 with 2 drives is too mental for my liking!

 

[WilliamG]

That was the point where my eyes went wide open and I thought of Harry Enfield "nooo, you don't want to do that".

If a client demanded he wanted me to setup his 5k booting OS X over tb2 in raid 0 I would have to shake his head vigorously till he changed his mind. A single drive is ok, a big array would be even better and faster but raid 0 with 2 drives is too mental for my liking!

Don't see what's wrong with RAID0 with 2 drives over TB2. Sounds speedy! As long as you have backups, what's the problem?

Ohh Harry Enfield. I grew up with that..."Noo, you don't wanna do that! You want that one!"

 

[Gav Mack]

I've been booting off Thunderbolt for a hair under two years now with never a single issue on my 2012 iMac. I'm just using a Seagate Backup Plus Thunderbolt enclosure with a 256GB Samsung 830 SSD, mind,

Single drive isn't raid 0 though - which is like running nitrous wondering if your Pistons are going to fly out your engine block.

Raid 5 or higher great, speed and redundancy built in.

 

[theSeb]

That was the point where my eyes went wide open and I thought of Harry Enfield "nooo, you don't want to do that".

If a client demanded he wanted me to setup his 5k booting OS X over tb2 in raid 0 I would have to shake his head vigorously till he changed his mind. A single drive is ok, a big array would be even better and faster but raid 0 with 2 drives is too mental for my liking!

Yeah, I would not boot off raid 0 period.

 

[Gav Mack]

Don't see what's wrong with RAID0 with 2 drives over TB2. Sounds speedy! As long as you have backups, what's the problem?

Ohh Harry Enfield. I grew up with that..."Noo, you don't wanna do that! You want that one!"

Speed is fantastic- I love it but raid 0 has no brakes!

 

[scottomfg]

That was the point where my eyes went wide open and I thought of Harry Enfield "nooo, you don't want to do that".

If a client demanded he wanted me to setup his 5k booting OS X over tb2 in raid 0 I would have to shake his head vigorously till he changed his mind. A single drive is ok, a big array would be even better and faster but raid 0 with 2 drives is too mental for my liking!

Yeah, you crack me up. Talk about risk averse, ha. Anyway, I practice very sound backup techniques, even a total meltdown on a RAID 0 TB2 boot drive would be like an hour hiccup in my day. No reason to live in fear over that outcome. Anyway, lets try to get back on topic: More info from people with 5k iMacs about the TB2 situation.

 

[scottomfg]

Yeah, I would not boot off raid 0 period.

Been booting off RAID 0 in various configs for a decade, never an incident that wasn't easy to fix. Including back to the bad old hdd days, I've only ever had one drive failure in RAID 0. Replaced drive, copied my backup back to it, and voila.

 

[theSeb]

Been booting off RAID 0 in various configs for a decade, never an incident that wasn't easy to fix. Including back to the bad old hdd days, I've only ever had one drive failure in RAID 0. Replaced drive, copied my backup back to it, and voila.

Fair play, but back on topic. Have you seen the latest couple of posts before we sidetracked into raid 0?

 

[Gav Mack]

Yeah, I would not boot off raid 0 period.

I've only mentioned the Mac booting off raid0. Must have been 20 odd PC gamers through the years who found out to their cost raid 0 booting windows was a very bad idea.