SATA3: 6 Gb/s Or 600 Mb/s? - System Building And Upgrading

#1 Taoki

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Posted 03 April 2020 - 05:35 PM

There's a paradox I cannot figure out: The theoretical speed of the SATA3 interface is presented to us as being 6 GB/s. Yet whenever someone discusses HDD / SSD benchmarks, they keep saying SATA3 is limited to 600 MB/s. 6 GB is 6000 MB not 600 MB! Who's right in the end; Is the optimistic 6 GB per second spec real, or were we wrong all these years and only had 10 times less?

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#2 pcpunk

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Posted 03 April 2020 - 07:13 PM

It's not GB.  6Gb/s is 600Mb/s Same thing.  I feel your pain, the tech community seems completely incomprehensible.

SATA 3 Gb/s port = SATA-II both the same but very confusing because there is no standardized way of saying it seemingly. 

Some will say SATA-III but some will say SATA 6Gb/s

SATA 6 Gb/s port = SATA-III

If I don't reply right away it's because I'm waiting for Windows 10 to Update.

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#3 Taoki

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Posted 03 April 2020 - 08:11 PM

I'm still confused though: If 1 GB is 1000 MB and not 100 MB, why is it being called 6 GB/s if it's 600 MB/s? I'm assuming it's a marketing gimmick? Though even for that it seems kind of a stretch, as it's pretty much a false spec... same as if someone used the term Gigabit to describe 100 MB/s networking cards.

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#4 jonuk76

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Posted 03 April 2020 - 08:13 PM

It's 6 Gbits/s which is roughly 600 Megabytes per second.  There's 8 bits in a byte, but according to Wikipedia (which I believe is accurate on this particular topic) - "Third-generation SATA interfaces run with a native transfer rate of 6.0 Gbit/s; taking 8b/10b encoding into account, the maximum uncoded transfer rate is 4.8 Gbit/s (600 MB/s)."

Real world I don't think I've seen anything get more than about 550-560 Megabytes per second, probably due to overheads in the SATA protocol etc..

Edited by jonuk76, 03 April 2020 - 08:16 PM.

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#5 Taoki

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Posted 03 April 2020 - 08:38 PM

Ohhh... I see it now: It's gigaBITS not gigaBYTES. That difference has gotten me confused several times in the past actually. I'm assuming others too, hence why in this case using the term "GB" isn't considering that most of us are used to perceiving it based on everyday experiences with interfaces where it refers to Gigabytes (disk space, network speeds, etc).

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#6 jonuk76

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Posted 03 April 2020 - 10:49 PM

That's right.  And watch for the capitalisation of the "B" as I believe technically, MB/s and Mb/s mean different things (the former being megabytes, the latter being megabits).  I would say network speeds in particular, but also serial protocols like SATA and PCI Express, speeds are nearly always quoted in terms of kilo/mega/giga-bits per second rather than bytes.  Storage space and RAM amounts on the other hand, it would very unusual to quote them in terms of bits.

Some common network standards are "Fast" Ethernet/100 BaseT (100 Mbits/s), Gigabit Ethernet/1000 BaseT (1000 Mbits/s), Wireless G (54 Mbit/s), Wireless N (150, 300 or 600 Mbit/s depending on number of antennas), Wireless AC (it's complicated).

And of course there is still a gap between theoretical maximum speeds and actual performance in real world conditions.

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#7 Chris Cosgrove

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Posted 04 April 2020 - 06:09 PM

@ jonuk76 #4 & 6

Thanks for the explanation Jon. It's something I had noticed over the years but I have usually had more important things to worry about than go tracking down an explanation.

Marketing comes into it too. For the marketeers, 1GB = 1000MB instead of, is it, 1012 MB which of course leads to all the questions on BC as to why does my shiny new 1TB drive only show up as 960 GB. And, as you said, with regard to internet speeds, 50 Mbps sounds so much better than 6MBps.

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#8 MDD1963

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Posted 04 April 2020 - 07:56 PM

I've yet to see the best SATA SSDs (860 EVO, Crucial MX500, etc)  break 563 MB/sec in sequential reads in CrystalDiskMark...(I was actually pleasantly surprised to even see that high, frankly)

Doubt many have seen any break into the 570 MB/sec 'barrier'...

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#9 Taoki

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Posted 10 April 2020 - 03:45 PM

Still got one more confusion here: I took a quick look at the Gigabit to Megabyte converter on DuckDuckGo. It seems to suggest that 6 Gigabits is actually 750 Megabytes... with different online converters giving me roughly more or less this amount, though the number always starts with 7. Are the converters wrong or don't apply to this case, or is 600 MB/s just rounding down the number for convenience while the real maximum speed could still be 700 MB/s? I'm assuming the encoding jonuk76 mentioned plays a role in this calculation, it doesn't seem to be simply explained for SATA3 from what I can find.

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#10 MDD1963

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Posted 11 April 2020 - 08:03 PM

I've yet to see a number higher from a SATA3-spec SSD above 563 MB/sec (my 7700K testing a new fresh Crucial MX500)  in sequential reads...; whether that limit is in SATA3 (or associated overhead), the SSD itself, or even chipset limitations, who knows. But, I've yet to see anyone break into even 600 Mb/sec sequential reads, much less 700-750, etc...

Asus Z270A Prime/7700K/32 GB DDR4-3200/GTX1060

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