nRF52840 USB MSD Slow Write Speed

RE: nRF52840 USB MSD Slow Write Speed

Turbo J — Thu, 25 May 2017 10:39:46 GMT

Modern OS may do MB-sized transactions, which have little to none status infos on-the-fly available. This will look like a "hangup" of roughly 10 seconds to the user. Note that a single "write" transaction has no real limit in the underlying SCSI specs. GB-sized transactions are possible in theory - if the device has that many bytes available.

RE: nRF52840 USB MSD Slow Write Speed

Radoslaw Koppel — Thu, 25 May 2017 10:15:49 GMT

Last time we was testing the MSC speed of the stack itself it was around 700 KB/s (using dd on EMPTY drive - 512MB of zeros).

Do not say that 177KBps is unusable for 8MB drive - it can be totally written in less than a minute. This is quite obvious that for bigger drive faster FLASH and background write algorithm should be used. The example here just shows how to use MSC class and we tried to keep it simple.

When it comes to the "halts" you have described, this may relate to PAN 104 and the library is resuming from "lost event" situation. This would be corrected in the final product.

RE: nRF52840 USB MSD Slow Write Speed

Fan Jiaming — Thu, 25 May 2017 10:08:51 GMT

Could this also explain why I observed a freeze during data transfer? Is it because the read and write are not processed at the same time, therefore they take their turns which appear as a freeze?

RE: nRF52840 USB MSD Slow Write Speed

Turbo J — Thu, 25 May 2017 09:09:54 GMT

very slow write speed with only 177 KB/s

That is not very slow. The maximum speed is around 1 MByte/sec for USB 2.0 full speed - but that would require quite some rather intelligent buffering with some more RAM, as you need to read from USB and write to memory at the same time.

The 177 kB/s is about the expected speed when you are not doing this at the same time. This keeps the source code much simpler to understand - especially when there are abnormal conditions like write errors.