Link Speed vs. Negotiated Speed

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Level 1

8 points

Link Speed vs. Negotiated Speed

Hello:

I recently added an external SSD to my iMac (Model 14,1) which is a late 2913 21.5 Inch system. I was very happy with the resulting performance boost. I did some testing with BlackMagic Disk Speed Test which yielded some interesting results. My external SSD on USB 3.0 ran about 380 MB/s write and 430 MB/s read. I also have an external Toshiba drive (3 TB 5700 rotational speed) on the USB 3.0 which yielded 130 MB/s write and 125 MB/s read. Finally I tested my internal HDD (1 TB 5400 rotational speed) which peaked at 70 MB/s read and write. I was surprised by these results as I expected the internal HDD to be faster than the external 3 TB HDD.

Looking at the system report I see that my SATA bus is an Intel series 8 chipset that is capable of 6 Gbps transfer rate but the negotiated link is 3 Gbps, which would certainly explain why my internal drive has the slowest transfer rates. My question is is this normal? Is the hardware behaving to spec or do I need to make an adjustment and or bring it in for service? I am pretty tech savvy so I can make adjustments if needed; just need to know how.

Thanks.

iMac, OS X El Capitan (10.11.6), 21.5 inch late 2013 8 GB RAM 1 TB H

Posted on Aug 17, 2016 7:40 PM

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Answer 1

den.thed

Level 10

92,380 points

Aug 18, 2016 5:18 AM in response to george287

Evening George:

Those numbers (including the doggy 5400rpm internal drive) all sound right on the money to me.

One missing number from your test is an SSD on Thunderbolt, which would come in over 490 MB/s write and around 520 MB/s read, depending on the SSD.

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Answer 2

george287 Author

Level 1

8 points

Aug 18, 2016 5:23 AM in response to den.thed

I was afraid of that.I don't have a thunderbolt drive because at this time they are way more expensive. Thing is that I would have expected my internal drive to have the fastest link but now I know that just ain't so. The upshot is that anyone who is looking to boost speed via an SSD or who is considering upgrading their HDD should go with an external option if their system has USB 3.0.. No point in going through the time and expense of replacing an internal drive IMHO.

Thanks for your help.

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Answer 3

den.thed

Level 10

92,380 points

Aug 18, 2016 6:45 AM in response to george287

george287 wrote:

I was afraid of that.I don't have a thunderbolt drive because at this time they are way more expensive.

I agree.

The upshot is that anyone who is looking to boost speed via an SSD or who is considering upgrading their HDD should go with an external option if their system has USB 3.0..

Agreed.

USB 3.0 is 5G, so you should actually be able to over 600 MB/s *depending on the enclosure and SSD.

see > http://www.macworld.com/article/2039427/how-fast-is-usb-3-0-really-.html

No point in going through the time and expense of replacing an internal drive IMHO.

Not easy and then only if one was replacing it with a SSD.

https://www.ifixit.com/Device/iMac_Intel_21.5%22_EMC_2544

Thanks for your help.

You're welcome.

380/430 are great numbers, what *external SSD or *enclosure and SSD are you using...?

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Answer 4

george287 Author

Level 1

8 points

Aug 18, 2016 7:13 AM in response to den.thed

SanDisk Extreme 500 489 GB portable SSD USB 3.0. At $149 it represented the best price/performance combination for what I was trying to do. I wanted to up my VMware performance without voiding my Applecare. It worked. The specs for this are 400 write/430 read. I got 380/400 so I am getting the full measure of my SSD through USB 3.0.

I have a SanDisk SSD in my older Macbook Pro which sports an older SATA interface (Intel Chipset 5 Series). Its speeds are 250/262 MB/s write/read. Its specs are 500/550 MB/s.

What I am getting at is I do not think that an internal drive will yield faster results.

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Answer 5

den.thed

Level 10

92,380 points

Aug 18, 2016 7:45 AM in response to george287

Thank you. Again, I think that those are great numbers.

I agree. Example: the Nvidia 3G in my 2010 Mac Mini with an SSD reports the same 250/260 MB/s speeds as your MacBook Pro.

george287 wrote earlier:

Looking at the system report I see that my SATA bus is an Intel series 8 chipset that is capable of 6 Gbps transfer rate but the negotiated link is 3 Gbps, which would certainly explain why my internal drive has the slowest transfer rates.

An SSD should get the higher speeds on that 6G chip set.

greorge287 wrote:

What I am getting at is I do not think that an internal drive will yield faster results.

Agreed... The problem is... is it the el cheap-o hard drive or the SATA cable that is dragging the negotiation speed down to 3G...?

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Answer 6

george287 Author

Level 1

8 points

Aug 18, 2016 8:03 AM in response to den.thed

Good question.

Either way I have to say that I am disappointed that Apple is using cheap parts in their machine as they charge premium prices. The cost difference for a faster drive or a better cable is negligible compared to the profit margin I assume that they are enjoying. Of course I do not know their actual margins but considering their stock price and the fact that the iMac is built on open architecture, which means that its parts are commodities, I am sure that their costs are very low. Their competition is able to sell the same bunch of parts, in a less flashy package, for 1/3 the price of a Mac and make a handsome profit doing so.

Tsk - tsk.

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Answer 7

den.thed

Level 10

92,380 points

Aug 18, 2016 8:18 AM in response to george287

Sad indeed...

Adding that, I may have miss spoke after thinking about it some more...

den.thed wrote:

The problem is... is it the el cheap-o hard drive or the SATA cable something else in the architecture that is dragging the negotiation speed down to 3G...?

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