Why internal SATA negotiated link speed only 3 gigabit and not 6 Gigabit

Question

dennis826 Author

Level 1

8 points

Why internal SATA negotiated link speed only 3 gigabit and not 6 Gigabit

This has been bugging me for a while:

My iMac:

Model Name: iMac

Model Identifier: iMac17,1

Processor Name: Intel Core i7

Processor Speed: 4 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 8 MB

Memory: 24 GB

Vendor: Intel

Product: 10 Series Chipset

Link Speed: 6 Gigabit

Negotiated Link Speed: 3 Gigabit

MY older 14,2 iMac is 6/6.

I mostly use external Lightning 2 drive which are 6/6.

Posted on Dec 21, 2018 11:45 PM

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Answer 1

Allan Jones

Level 9

78,134 points

Dec 22, 2018 9:13 AM in response to dennis826

You'll need to ask Apple "why." We are all end users like you and can only quote history and our observations, not reasons why Apple does or doesn't do anything unless they publish the information.

Observation: They moved iMac logic boards to the 6GBps SATA III drive interface in mid 2011 but continued to install 3GBps rotational drives in some models. This is from our 21.5-inch Mid 2011 iMac:

That drive is a 5400RPM model so that computer tend to load things slowly.

I bought our daughter a new 2017 21.5-inch 4K 3.0Ghz i5 a year ago and this is the System Information section on her SATA bus:

The specs I have show that all the 2015 iMac 27s have at least a 7200 rpm drive so I doubt you are going to suffer a noticeable speed difference.

If you wish us to further cogitate on this, you can post a screenshot of what System Information shows for your SATA/SATA Express bus. That will show the Apple part number for your hard drive and we should be able to look up its specs.

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Answer 2

dennis826 Author

Level 1

8 points

Dec 22, 2018 7:30 PM in response to Allan Jones

Ok, went to Best Buy today to pick up a small SSD and while I was there I checked out their iMac 21.5 and 27. The 21.5 had the same 6/3 while the 27 was 6/6. They use the same chipset but different 1TB hard drives. So my guest is the seagate drive in my system is not setting the link speed to 6GB.

As I said earlier, I saw this when I first purchased the 17,1 iMac (a couple of days after availability) buy was not concerned because I use the internal drive for recovery and backup storage. My main work uses Thunderbolt Enclosures and they do not show this "problem".

If I still was friendly with someone at Apple I would ask them but as I am retired and I have not maintained my Apple contacts.

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Answer 3

Dec 22, 2018 10:58 PM in response to dennis826

https://eshop.macsales.com/item/OWC/K27IM12HE2TB/

you might consider replacing the internal with a 6G SSD...if speed was an important consideration to you.

I understand the frustration of expecting a certain performance spec and discovering that the product does not meet that expected spec, due either to misunderstanding or incomplete/misleading information. Booting and launching apps from the 6G SSD drive would be faster than the 3G platter drive (5400 or 7200 RPM). If you are just running backups, does it really matter how long that takes?

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Answer 4

DonH49

Level 6

16,716 points

Dec 22, 2018 4:43 AM in response to dennis826

the most important specification you don't mention. Is the internal drive a fusion, a platter or an SSD? The 6 gig connector is most likely inside but not connected. I believe if you research it's doable.

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Answer 5

Dec 22, 2018 8:08 AM in response to dennis826

Look at the speedometer in your car. Chances are it indicates speeds of above 100 MPH. Does this mean you are guaranteed to reach that speed? No. The 6 GB/sec is a theoretical maximum speed, but you are not guaranteed to get that at all points in time. Every communication protocol has 'overhead' that would prevent you from obtaining the full speed. On a 1 GB/sec ethernet connection you may see 900 MB/sec or even 950 MacBook/sec, but never, ever the full 1 GB/sec because of this overhead. For whatever reason, your system is auto-negotiating down to a relatively stable 3 GB/sec. Not ideal, but not being a slowpoke either.

I hope this helps you understand the situation.

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Answer 6

dennis826 Author

Level 1

8 points

Dec 22, 2018 9:07 AM in response to Buster_From_Oak_Park

Let me add, I have been in the computer industry writing application code code, designing hardware and writing operating systems. I said I was bugged by the fact. I never expect rotating media to even reach the original SATA transfer specification on a large transfer (too much head movement and latency). Which is an important fact I left on in my original post. To maximize transfer speeds to my SSDs, I connect them to my Thunderbolt 2 cable and use the internal drive for backup and recovery.

I was just bugged that the Seagate drive and the Intel chipset are suppose to be able to run at 6GB; If anyone new which was not not allowing the maximum link speed (I suspect Seagate). I do not intend to open up the unit to change out the drive. I probably will trade this unit in for a New iMac once the start using the current Intel Processors. If I was a rich, I might think about an iMac Pro. I also own 2 $10K Lisas.

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