18 April 2019

Slow file copying in a Windows 10 network

A friend of mine was lamenting today that his network file copies were taking minutes to complete. Initially, I thought it was just a case of 21C impatience, but he showed me that files that only weighed in at several hundred KB were taking up to a minute to transfer to his other machine. It used to be fine, so something in the system changed, and obviously the fact that Windows 10 updates itself when it wants to makes it easy to blame. This friend was tearing his hair out. He'd tried a bunch of different things, including rebooting the router and making sure his ethernet cables were securely seated (he only runs on wired networks, not trusting the ether). He even had a contact - apparently a network-savvy guy - do a TeamViewer session, but all the changes they made amounted to nothing.

I tracked down the solution that fixed the problem pretty efficiently after I discounted the chaff that always turns up when you search the web, including several answers that didn't help from Microsoft. Here it is:

  1. Right click on your Windows logo and select Device Manager from the menu;
  2. You will get a list of all devices on your machine. Expand Network Adapters;
  3. Find your Network Card and double-click on it;
  4. Select the Advanced tab. You will get a list filled with different options;
  5. Select Large Send Offload V2 (IPv4) and set the value to Disabled;
  6. Do the same for Large Send Offload V2 (IPv6) if it is available
  7. Click OK
I got my friend to do the same on both computers, just to make sure that neither was attempting to use LSO. The result is immediately evident, but I've asked my friend to try copying some large files over as well as the small ones in case they want the LSO re-enabled.

My friend reported back that a 3 GB zip file copied over in about 49 seconds and a folder of 451 TGA images totalling roughly 1.2 GB copied over in 39s - both totally acceptable times. Then he copied 157 GB of image data:

Before changing these LSO values for both machines, this operation could have taken days.

The original source for this information from an old article here, but the LSO information is still pertinent today.

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