Realistic SSD Throughput

Oldmanmac

macrumors 6502
Original poster
Mar 31, 2012
441
14
Edmond, OK
I Have a 2015 27"i 3.5 GHz iMac. With ALL the talk of SSD's speeding things up, what can I realistically expect from a SSD (external) TB2 drive? Is the SATA 3 the limiting factor? I am ignorant enough to be dangerous.Help!
 

MacUser2525

macrumors 68000
Mar 17, 2007
1,773
195
Canada
All the talk is when they are installed internally. When in external enclosure they are always limited by the speed of the external bus interface used, the directly connected internal drive will always beat the external for this reason.

Edit: The realistic outcome can be easily tested take any drive connected to the TB2 in an enclosure and speed test it, the test will be the same no matter the type of drive used. As the eternal interface will never give you more than an your already connected internal drive. In short it will always be slower connected externally.
 

kohlson

macrumors 68000
Apr 23, 2010
1,986
537
This is a complex subject. But speed is usually considered as "getting things done faster." In a pure file transfer, the interface and/or the media can be a determining factor. For many, many OS and app related things, IOPs/sec is the determining factor. TB2 is a pretty fast interface - way faster than SATA3. If the drive/interface inside the TB2 enclosure is fast (beyond the SATA3 6Gbps) then it will be faster than a system running from an internal HDD.
 

mikehalloran

macrumors 68000
Oct 14, 2018
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The Sillie Con Valley
No one has answered your question. The answer is it depends.

A single SATA III is limited by that spec no matter what the enclosure or how connected—exactly as you suspected.

You can get a RAID enclosure to approach or slightly surpass the internal speed with four NVMe PCIe 2 SSDs. Very expensive.

You haven't asked the right question, however.

q. What is the best way to increase speed and add storage to your 2015 iMac?


a. Open it up and replace the SSD on the PCIe buss.

Apple installed a PCIe 2 SSD into the PCIe 3x4 buss on the 2015 iMac. You can double the speed with up to 2TB storage on that buss by installing a 970 EVO (NVMe PCIe 3x4) SSD with an inexpensive pin-out adapter. Under $600 for the parts plus an hour labor.

This gives you the equivalent of a 2017 iMac 2TB SSD except that it has TB2 ports instead of TB3/USB-C

If you want, when it's open, you can install a SATA III drive up to 4TB on the other buss. Yes, you can achieve the same with an external but it's less expensive to just add it at the same time you're giving that 2015 its Wheaties.
 

psymac

macrumors 6502a
Jul 17, 2002
508
114
No one has answered your question. The answer is it depends.

A single SATA III is limited by that spec no matter what the enclosure or how connected—exactly as you suspected.

You can get a RAID enclosure to approach or slightly surpass the internal speed with four NVMe PCIe 2 SSDs. Very expensive.

You haven't asked the right question, however.

q. What is the best way to increase speed and add storage to your 2015 iMac?


a. Open it up and replace the SSD on the PCIe buss.

Apple installed a PCIe 2 SSD into the PCIe 3x4 buss on the 2015 iMac. You can double the speed with up to 2TB storage on that buss by installing a 970 EVO (NVMe PCIe 3x4) SSD with an inexpensive pin-out adapter. Under $600 for the parts plus an hour labor.

This gives you the equivalent of a 2017 iMac 2TB SSD except that it has TB2 ports instead of TB3/USB-C

If you want, when it's open, you can install a SATA III drive up to 4TB on the other buss. Yes, you can achieve the same with an external but it's less expensive to just add it at the same time you're giving that 2015 its Wheaties.
Yes, I have ordered these parts, and will be doing this upgrade soon for both a 2015 and 2017 27”. Will post before and after R/W results. Still on the fence of also replacing the HD with an SSD, as the 970 Evo will be 1TB size.
 

Fishrrman

macrumors P6
Feb 20, 2009
17,160
5,520
OP:

Do you really need thunderbolt?
Could USB3 do the job for you?

USB3 external SSD's are quite fast and are cheaper than thunderbolt.

A USB3 SSD (that supports UASP) should give you read speeds at least 420-430mbps and writes from 300-350mbps (depending on drive mfr and size), or even faster.
 

kohlson

macrumors 68000
Apr 23, 2010
1,986
537
You can get a RAID enclosure to approach or slightly surpass the internal speed
Can you expand on this? It's my understanding that the interface determines the absolute maximum transfer rate. Latency can be reduced when using RAID.
 

mikehalloran

macrumors 68000
Oct 14, 2018
1,600
390
The Sillie Con Valley
Can you expand on this? It's my understanding that the interface determines the absolute maximum transfer rate.
Yes but...

RAID can use multiple drives to increase speed or to increase storage depending on how it's configured—it's a bit complicated but there are charts that show what flavors of RAID do what.

The PCIe buss it connects to determines the max speed.

With the 2015 model iMac, the Apple PCIe 2 chip throttles the internal speed to 1500 RW. The 2015 buss is PCIe 3x4 and rated at twice the speed—if it's getting something faster externally, it's up for that (this is why installing a quad chip like the 970 EVO gives a 2015 its Wheaties).

The OWC RAID, when loaded with four NVMe PCIe 2 chips, is rated up 2200 RW if configured in a version of RAID designed to increase speed. Let's see, that's $349 for the empty housing plus up to 4 chips @ $... It adds up.
 

goslowjoe

Suspended
Dec 22, 2017
125
88
If you want, when it's open, you can install a SATA III drive up to 4TB on the other buss. Yes, you can achieve the same with an external but it's less expensive to just add it at the same time you're giving that 2015 its Wheaties.
Mike, I think you have answered here my question in a thread I created earlier about adding a SATA III SSD to a Late 2015 27" iMac with a PCIe SSD already installed. It will be awesome if I could have the SSD added internally - much faster than through the USB port and way neater than having something hanging off the back of the iMac.