SATA Definition MB/s and Gb/s

[sso]

Hi,

Can someone with insight into SATA Specs confirm the following in relation to SATA transfer speed definitions:

MB/s is MegaBYTES/second
Gb/s is GigaBITS/second, such that:

150 MB/s = 1.2 Gb/s
300 MB/s = 2.4 Gb/s
1.5 Gb/s = 187.5 MB/s
3.0 Gb/s = 375 MB/s

Thanks.

Stephen Odgaard

 

[Crashman]

The problem is, when you do the math right you don't come up with the same numbers as the drive makers, who would call 300MB/s 3 gigabit, even though it isn't.

<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>

 

[lyberty]

{Correct one's are...}
1.5 gbps = 187.5 MB/s
3.0 gbps = 375 MB/s
[/unquote]

Well, yeah, _you_ say so, but do you have a reference?

Not that we distrust you; it's just good to get it externally verified...

Another issue:
what metrics are used for the Tom's hardware guide performance benchmarks for HDD's?

Looking at
http://www.tomshardware.com/storage/20051003/western_digital-03.html#transfer_performance
for example:

the top read speed in the "Data Transfer Diagram" (top of page) looks to be about maybe 63,000 to 64,000 KB/s.

So why does the "Write Transfer Performance" chart
show 64.6 MB/s maximum for the Western Digital WD4000KD??
Shouldn't it say "62.5" (64,000 divided by 1,024)?

There should really be a key on these diagrams (e.g. "Note: 1 MB/s = 1024 KB/s" or "1 MB/s = 1000 KB/s"

 

[lyberty]

Right you are!

But the question is: are the reviews showing
- MiB/s (the new name for 1024 KB)
- MB/s (the flip side of MiBs, where the IEC has redefined 1 MB as 1000 KB), or
- MB/s (the definition accepted over 20+ years, where 1 MB is 1,024 KB)
?

 

[lyberty]

Okay, here's the problem.

If you're talking about data transfer rates for Hard Disk Drives,
you need to use BITS in your discussion. This is because you are measuring PULSES (hertz and bits). [A megabit is a million binary pulses, or 1,000,000 (that is, 10^6) pulses (or "bits").Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system.]

But if you need to switch to Bytes for some reason, you are now talking about File Transfer speeds, which is a measurement of how fast you can move BYTES (not bits). For example, "how long will it take to transfer this file that is 200 MB?" Now you are in the world of base-2 (/1,024), because your computer is using the binary number system for Storage. So now you're asking (using the previous example) "How long to transfer 209,715,200 bytes?"

Note that SATA-IO refers to BIT-RATES for precisely this reason [1.5Gb/s and 3.0Gb/s].

In summary:
Discussion of data transfer rates are in bits, even if you are talking about file transfer speeds.
If you need to refer to Bytes for some reason, you must switch to base-2 (/1,024),

Therefore:
1.5 Gbps* = 187,500,000 Bytes per second = (approx) 178.81 MB/s.
and
3.0 Gbps* = ((3*1,000,000,000) /8) /1,048,576 = (approx) 357.63 MB/s.

You may not "give a damn", but it affects your answer.

So a different answer would be:

These are the correct ones:
1.5 Gb/s = 179 MB/s
3.0 Gb/s = 358 MB/s

See what I mean?


*"Gbps": Billion bits per second, a.k.a. giga-bits per second.

References:
<A HREF="http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci212534,00.html" target="_new">http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci212534,00.html</A>

<A HREF="http://www.lyberty.com/encyc/articles/kb_kilobytes.html" target="_new">http://www.lyberty.com/encyc/articles/kb_kilobytes.html</A>

 

[lyberty]

wusy-

I understand your point that as long as the metrics are consistent within the review, then the comparison is valid.

But my point was that your answer to "sso" (at the top of this thread)
is incorrect.

SATA 1.5Gb/s (aka SATA/150)

1,500 MHz embedded clock
x 1 bit per clock
x 80% for 8b10b encoding
/ 8 bits per byte
-----------------------------------
= 150 million Bytes per second
in other words:

150,000,000 Bytes per second
/ 1,024
----------------------------------
= 146,484.375 KiloBytes per second
=~ 146,484 KB/s

146,484.375 KiloBytes per second
/ 1,024
----------------------------------
= 143.0511474609375 MegaBytes per second

= 143 MB/s

(not 187.5 MB/s).

Regards, Lyberty

----
<A HREF="http://www.lyberty.com/encyc/articles/kb_kilobytes.html" target="_new">http://www.lyberty.com/encyc/articles/kb_kilobytes.html</A>

 

[lyberty]

Thanx for the correction. [/geek]

Actually, I prefer "nerd"

And you know these are the message boards for Tom's hardware guide, right? It doesn't get much more geeky that this ... ;-)