Difference between 6GB/s and Read / Write speeds?

AntaresX

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I don't understand why SSDs are advertised as being 6.0GB/s yet only being able to Read/Write ~500MB/s. What's the difference between these two specifications and if the SSD can only push 500MB/s why does it matter if the SATA III is capable of 6.0GB/s?
 


You are right technically with regards to your math. 6Gb/s = ~750MB/s.
But if the conversation is regarding SATA speeds then you are incorrect.

The way SATA ports are designed the maximum data bandwidth of a SATA 3.0 (6Gb/s) port is ~600MB/s, not ~750MB/s.

The difference is due to protocol overhead (10b/8b coding with 8 bits to 1 byte). Don’t ask me to explain what it means, it’s over my head. LOL

The maximum data bandwidth of a SATA 2.0 (3Gb/s) port is ~300MB/s, not ~375MB/s.
The maximum data bandwidth of a SATA 1.0 (1.5Gb/s) port is ~150MB/s.
 

aerai

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The SATA-III connection is a 6.0 Gb/s (small b) which is bits. When converted into bytes (big B) there is a ratio of 8 bits to a byte. So:

6 Gb = 6000 Mb = 750MB (roughly speaking)

The previous standard, SATA-II was 3.0 Gb/s which is about 375 MB/s and so not capable of running the faster SSDs. I think there is a bit of variance in the actual transfer rate though which means they would likely be slower than those I have used as examples.
 

RealBeast

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It matters for SSDs but not for HDDs. The SATA 6Gbps ports will transfer data faster than current SSDs are capable of reading or writing, but the 3Gbps will not, as many SSD speeds exceed that limit.

Recall that you are talking about gigaBIT port speeds and megaBYTE data transfer rates on SSDs.
 


6Gb/s is the maximum data transfer rate of a SATA Revision 3.0 (SATA III) port.
SATA III speeds are from 301MB/s to 600MB/s.

So when you see SATA 6Gb/s advertised that is the interface that the manufacturer is referring to. The actual performance of the SSD can be from 301MB/s to 600MB/s.
 

AntaresX

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Ah, that makes perfect sense now! Thank you for the simple explanation. Even with a bit of variance I imagine they'd still allow the fastest SSDs out there to reach their maximum speeds.
 


You are right technically with regards to your math. 6Gb/s = ~750MB/s.
But if the conversation is regarding SATA speeds then you are incorrect.

The way SATA ports are designed the maximum data bandwidth of a SATA 3.0 (6Gb/s) port is ~600MB/s, not ~750MB/s.

The difference is due to protocol overhead (10b/8b coding with 8 bits to 1 byte). Don’t ask me to explain what it means, it’s over my head. LOL

The maximum data bandwidth of a SATA 2.0 (3Gb/s) port is ~300MB/s, not ~375MB/s.
The maximum data bandwidth of a SATA 1.0 (1.5Gb/s) port is ~150MB/s.
 

AntaresX

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Right, right. That last sentence is where I got really confused. I bet most people wish SATA III was capable of 6 gigaBYTES per second, since it would probably future proof them in regards to upcoming SSDs for at least a decade.
 

AntaresX

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Understood. Is that upper limit of 600MB/s mainly due to the limitations of SATA III? Or is that just where SSDs are at the moment?


 

aerai

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I knew there was something that put the values down and have seen that 10/8 before but wasn't sure on it. Makes the maths a bit easier though since we can just divide by ten. Thanks for the info.
 

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