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I'm getting only 30MB/sec between my computer and a USB drive, despite the fact that USB 2.0 supports 480Mb/sec (or 60MB/sec) transfers. (Therefore, I'm only getting half the rated speed) Is there something present in the USB standard which should result in such half apparent speeds?

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Maximum typical speed? Isn't that an oxymoron? –  Mehrdad Jul 31 '11 at 4:56
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@Mehrdad: Not really. Max typical speed for a gigabit lan transfer is in the 100MB/s range, even though the theoretical speed is higher. –  Billy ONeal Jul 31 '11 at 4:57
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Oh then you mean maximum practical speed, right? –  Mehrdad Jul 31 '11 at 4:58
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@Mehrdad: Same thing. Yeah. –  Billy ONeal Jul 31 '11 at 4:58
    
Do you have any kind of compression or maybe multiple virus scanners running? Try disabling them. Make sure the cable is rated for USB 2.0. If it is old, it may be a slower-rated cable. I assume you would have said something if it were going through a USB hub. Is it slow for other devices, like a flash drive (you would need to check its rated transfer rate). –  KCotreau Jul 31 '11 at 5:00

8 Answers 8

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Your flash drive is the bottleneck. They can't reach the 60 MB/s theoretical maximum. Here's an excerpt from Wikipedia:

Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s (60MB/s) which the USB 2.0 Hi-Speed specification supports because of technical limitations inherent in NAND flash. The fastest drives currently available use a dual channel controller, although they still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB throughput. (...)

Typical fast drives claim to read at up to 30 megabytes/s (MB/s) and write at about half that speed. This is about 20 times faster than USB 1.1 "full speed" devices which are limited to a maximum speed of 12 Mbit/s (1.5 MB/s).

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It's not a flash drive. I'm copying to this -> amazon.com/Western-Digital-Passport-Essential-Portable/dp/… -- I'm asking what the limit of the interface is though, not about a specific device. (I want to know if the device is hitting the practical limit; that's the reason for this question in the first place) –  Billy ONeal Jul 31 '11 at 5:06
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Sorry, I misinterpreted your question. You didn't specify and 30MB/s is the usual speed for flash drives so I thought you were using one. –  nmat Jul 31 '11 at 5:10
    
I see. On the other hand, I didn't really ask what the bottleneck is, did I? :) –  Billy ONeal Jul 31 '11 at 23:46
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Honestly, I don't think a standard has limitations. A standard has a theoretical maximum and the implementation usually caps it. So if you want to know the cause of the slow speed, you have to look at the implementation. In case of flash drives the problem is in the NAND flash and in case of hard drives the problem is in controller logic. Even high speed controllers barely exceed 30MB/s. –  nmat Aug 1 '11 at 0:25

Around 30 MB/sec is quite typical maximum transfer speed.

USB 1.0 and USB 2.0 connections are half-duplex, meaning data flows in only one direction at a time. Shared connection between both directions is probably biggest reason for slowdown than expected transfer speed.

In comparison, USB 3 and Ethernet are full duplex and do meet expected transfer speeds better.

In my machine, an USB2 flash drive speed never exceeds 33 MB/s in test application, even though Windows reported 33-37 MB/s speed. I did some testing and enabled disk cache (device properties) and increased usb max transfer size to 2 MB (KB2581464) but could not make it any faster.

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With an iMac mid-2007 and one Verbatim USB2 disk transferring data to a FW800 drive I get 36-37 MB/s. It's already very good for USB2.

If I add a second transfer from another USB2 disk (Packard Bell) connected to the same USB2 hub to the same FW800 drive, the combined transfer rate increases to 42 MB/s. This is exceptional and it's the highest transfer rate I have ever seen on USB2.

More than 35-40 MB/s on USB2.0 is practically impossible and I was already dedicating a USB2 controller only for those disks, no mouse or other devices interfering.

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The USB 2.0 interface can be a limit due to signalling and command overhead as well as spacing between packets.

I have a fast SSD connected by USB 2.0. The drive is much faster than the interface (by more a factor of 10).

Read Speed maxes out around 33 MB/s and Write Speed at 17.5 MB/s. Write Speeds are almost 50% slower due to a verify-read after the write and the fact that the USB signal is half-duplex as another answer mentions.

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That sounds like an issue with your controller. I often see write speeds much faster than 17.5 MB/s, even with comparatively cheap drives, over USB 2.0. –  Billy ONeal Oct 26 '13 at 7:54
    
Do you have write caching on? I'm talking actual raw numbers running a disk benchmark program using write with verify. –  Adisak Oct 28 '13 at 23:12
    
Disk benchmarking programs often use unrealistic parameters for these USB controllers. USB has a very high per-object or per-transfer overhead associated with switching into bulk mode. Copying of large files to the USB drive doesn't run into this problem. Write caching couldn't have too much of an effect given that I was able to immediately put the drive in another machine and the data was not corrupt. –  Billy ONeal Oct 28 '13 at 23:17

I have never really thought much about calculating the speed, but clearly there is real overhead associated with this kind of transfer. I searched on Google and found post after post with speeds as you described, making me thing you are on to something.

I just whipped out a USB 2.0 1TB Seagate external drive, formated it, and decided to copy a sampling large enough to test with: 13,595,211,905 bytes (about 12GBs). I am running Symantec Endpoint Protection AV.

According to this calculator, it should have taken only 3:46 minutes to copy with 0% overhead, but it actually took 9:17, and my speed dropped to 23.9 MB/sec actually.

I then rebooted (to clear the memory), and tried it without my AV running and it still took 9:15, or only 2 seconds less (I guess that is good news for Symantec AV at least).

It would appear that those really are "theoretical numbers".

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P.S. The 480Mb rate is for all devices on a hub, but in my tests, it was really just the drive and mouse, and I doubt the mouse had a great impact. –  KCotreau Jul 31 '11 at 5:44
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About 24-26MB/s is the speed that I've seen on quite a lot of devices using USB2.0 –  Sathya Jul 31 '11 at 5:49

USB 2.0 supports 480 Mb/s signaling speed. On the Wikipedia page, it says effective throughput is up to 35 MB/s. There is a disparity because bits aren't usually transmitted between devices in the same way that they are represented internally. A number of factors needs to be accounted for when transmitting data between devices, like electromagnetic interference.

http://en.wikipedia.org/wiki/8b/10b_encoding

The link above is an example of an encoding scheme. It is used by USB 3.0.

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8b10b is a 20% overhead. 20% off of 480Mbits is 384MBits is ~48 MB/s. Still significantly faster than what I see in practice anywhere. –  Billy ONeal Oct 26 '13 at 7:53

yes, tipically usb 2.0 reach up to 30-34 mb\s writing, but with asrock xfast usb (software) I reached 44 mb\s writing speed with an external hdd WD Green 1 TB 32 mb cache (i think sata 2) !! i don't receive money from asrock, but simply i didn't belive it works before tryng it, I said that because it demostrate that the speed of 30-35 mb\s is due also to software bottleneck problem, i don't know more...

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Any chain is only as strong as its weakest link. The potentially weak transfer performance when using USB2 devices include everything from the host controller firmware, driver software, physical cables used (shorter and thicker may be slightly better?) and probably most importantly the actual connected devices themselves and any firmware, flash/cache chips and most definitely spinning disk performance, a well known bandwidth bottleneck.

Check in logical order of available I/O bandwidth - Processor/controller performance / L1/L2 Cache DRAM performance / firmware / flash storage chip performance then by an order of magnitude slow spinning disk storage performance.

Stated theoretical maximum performance (480 megaBITs or 60 megabytes per second) is only for the bus not things connected to it and quite often the actual observed performance is much lower.

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