Re: Slow transfer speed to USB 3.0 portable drive

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On 16 Aug 2024, at 19:28, Roger Heflin <rogerheflin@xxxxxxxxx> wrote:

It would depend on the buffer size being used. 

The buffer size is not the limit., unless it's lots of GiB in size.
It's the fact that you go into a half-duplex mode that sets the limit.

This pattern slows down all sorts of algorithms, disk access, network access etc is this:

1. Write
2. Wait for ack to write (sync)
3. Write
4. Wait for ack to write (sync)

It's the wait times for 2, 4 etc that kill performance even if you have infinite bandwidth.

This is why the early web was slow. Load a page that needs to load images etc
triggers this pattern.

If the user app can write while the kernel is flushing to storage then you get to run at
the limit of the storage interface. This is like full-duplex networking.

Barry

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