USB, SATA Bottleneck

[IceCreamBarr]

Can anyone help explain why USB and SATA specs state one transfer speed and yet we never see these speeds achieved. Ive done my googling and i cant even find a standard speed mentioned... i see 3Gbit and 300MB which are the same using 1000 bit to Mbit but when talking wireless speed or USB they mean 8 bits to a byte, ex. USB 480Mbit = 60MB. Now weve all seen harddrives connected externally to a USB and they max out at 35MB ish... where is the other 25... we know the drive can do it. And maybe SSDs cant do 300 MB yet but for sure the 32MB of RAM they are using for burst can achieve this... same question, where is the rest.

Thanks for the clarification.

Barr

 

[angelkiller]

Welll, most interfaces like SATA and USB have a maximum theoretical bandwidth. This is the speed that you never see. (480Mbps, 3Gbps, etc) These exist because purely for definition. You have to designate a top speed. Outside of that, there's no real purpose of the number.

The reason the theoretical number is so high is that it assumes 100% efficiency. But of course, this doesn't happen in real life. So, "if everything works perfectly" USB can reach 480Mbps. But as you already the actual max bandwidth is much lower.

Usually, 8 bits = 1 byte, but there's something wierd about Sata that makes it different. 3Gbps = 300MB/s for Sata, but Gigabit Ethernet (1000Mbps) provides a theoretical max of 125MB/s.

I don't get what you mean about your "where's the rest?" question about SSDs. Most SSDs, can only sustain ~230MB/s read. It doesn't reach the full 300MB/s because the flash chips arn't that fast yet. I don't really pay attention to burst becaue it has little effect on performance imo.

Maybe that answers some of your questions. I feel like I didn't answer them completely though.

 

[Mussels]

Can anyone help explain why USB and SATA specs state one transfer speed and yet we never see these speeds achieved. Ive done my googling and i cant even find a standard speed mentioned... i see 3Gbit and 300MB which are the same using 1000 bit to Mbit but when talking wireless speed or USB they mean 8 bits to a byte, ex. USB 480Mbit = 60MB. Now weve all seen harddrives connected externally to a USB and they max out at 35MB ish... where is the other 25... we know the drive can do it. And maybe SSDs cant do 300 MB yet but for sure the 32MB of RAM they are using for burst can achieve this... same question, where is the rest.

Thanks for the clarification.

Barr

SATA II is 3Gbit, which comes out to 375MB/s.

75MB/s is used for signalling and error correction, leaving 300MB/s for the drives.

USB 2.0 is 480Mb or 60MB/s.
However, like wireless technology some marketing genius decided to market the bandwidth in both directions at once - it can do 30MB/s each way, so lets call it 60MB/s!

(even tho the theory is you could get 59.9MB/s one direction and 0.1MB/s the other, due to the way it works you pretty much get capped at half each. 30MB/s is pretty much the norm for USB2.0, although some exceptional USB controllers and devices can do around 35MB/s)


Finally, these are just the maximum theoretical numbers as angelkiller stated. The drives themselves cant go as fast as sata II, so thats a limitation there - and then you have things like latency from long cables, EMI, bottlenecks elsewhere in the system (say, the CPU is too busy doing one thing, to process the requests of the SATA controller)

 

[IceCreamBarr]

Gotcha Truthful misrepresentation - the marketers scapegoat

Angel - what i meant by "the rest" is that the flash chips may or may not be able to hit the full 300MB but surely the RAM can hit that mark easily. When seeing tests of the peak throughput for burst, isn't an HDD or SSD transfering whatever is buffered in its own RAM (the 32MB part)?

 

[Mussels]

When seeing tests of the peak throughput for burst, isn't an HDD or SSD transfering whatever is buffered in its own RAM (the 32MB part)?

exactly right.

They use one small high speed cache to reduce the delays that would otherwise happen as the drive seeks around.

Drives can read really fast - its just that they have a delay between each bit they read, as they spin around.

 

[angelkiller]

SATA II is 3Gbit, which comes out to 375MB/s.

75MB/s is used for signalling and error correction, leaving 300MB/s for the drives.

Source? Never seen this before.

 

[a_ump]

source is mussels, a very trusted member here . i take his word for it personally. thx guys, i stumbled upon this thread randomly but it was interesting read and learned something lol

 

[Mussels]

Source? Never seen this before.

http://en.wikipedia.org/wiki/Serial_ATA#SATA_3_Gbit.2Fs_.28Second_generation.29


being wiki it doesnt say it simply and clearly, but you can see that they call it 3Bit and 300Mbyte at the same time.


the simplest they get is this

First-generation SATA interfaces, now known as SATA 1.5 Gbit/s, communicate at a rate of 1.5 Gbit/s. Taking 8b/10b encoding overhead into account, they have an actual uncoded transfer rate of 1.2 Gbit/s.