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Old 06-08-2019, 07:19 PM   #19
BernH
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Quote:
Originally Posted by ulyssesvideo View Post
I am referring to these portable drives from samsung...

https://www.ramcity.com.au/data-stor...e/MU-PA250B_WW

I am sure they will produce a usb4 drive as well.
The conversion factor from bits to bytes is 8. (B=Bytes, b=bits) This is derived from binary code where there are 8 bits in a Byte.

Those drives are listed as 540MB/s (the limit of the actual SSD in the housing), which using the conversion factor equates to 4320Mb/s. Divide by 1000 to convert to Gigabits is 4.32Gb/s (close to the 5Gb/s bandwidth of USB3) That's a far cry from 40Gb/s.

It would take a drive that is 9.25 times as fast to saturate a 40Gb/s bus. Plugging it into a usb 3.1 does not make it run at 10Gb/s, nor would a USB 4 connection make it run at 40Gb/s. It can only run at the slowest speed of any device in the chain (in this case 4.32Gb/s) when the chain is a serial one. If it is a parallel setup like a RAID-0/stripe set, the 40Gb/s is distributed across the multiple drives.

So once again, until a single drive can provide the bandwidth, you are looking at a drive that can not give the bandwidth that the bus can handle. The way around it is a stripe set of drives to handle a portion of the bandwidth each.

At some point, drives may reach the speeds to saturate the usb4 bus but by that time we will probably be several USB generations beyond that limit, and the same scenario will still apply.

This is just the simple math on how this stuff works.


EDIT: I should also note that even though USB3, as I noted above has a 5Gb/s bandwidth specification, often computers can not reach that limit, especially if the drive controller and USB chip being used do not support UASP (USB Attached SCSI Protocol) which allows the USB controllers to operate in a different mode that is does away with all the USB packet transfer overhead. Without this UASP mode, most computers will top out somewhere in the 3Gb/s to 4Gb/s area.
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Last edited by BernH; 06-09-2019 at 01:40 AM. Reason: fix typo
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