[Samba] Abysmal performance with Samba 4 and Windows Explorer

[Simon Schneider]

I'm experiencing extremely slow performance when trying to copy files from
and to a samba 4 share. I'm using Debian and the Samba 4 SerNet packages
and everything is up to date on the client as well as on the server. The
connection is made through 100Mbp/s ethernet, but the problem also exists
on a local virtual machine that runs on the Debian server.

By extremely I mean really abysmal:

* Copying a folder with 50 files total size 10 MB to Win7: ~ 20 minutes at
3-4 Kb/s
* Copying the same folder to Windows Server 2003: ~ 3 seconds
* Copying the same folder with ROBOCOPY to Win 7: ~3 seconds
* Copying a large random file 500 MB to Win 7: Takes ages to start copy
progress than up to 10 MB/s
* Copying the same large file to Windows Server 2003: Starts immediately to
copy and takes a few seconds
* Copying the same large file with ROBOCOPY using Win7: also decent
performance, no delays

I already googled for days and found a lot of suggestions to:

* Changes switches: one of the test machines runs as VM on host, so this is
not the issue
* Update network drivers: same thing, using different machines and a VM the
problem exists on all of them
* Turn off Remote Differential compression on the client: Didn't help at all
* Disable autotuning on the client with netsh: Didn't help at all
* Make sure not to use mapped drives but UNC paths instead: Did help when
logging in but has no effect on file transfer
* Try switching max protocol on the samba server to something like NT1: did
not help
* Do not use any other custom smb.conf options like socket etc.

What is the big difference between using Windows File Explorer and
Robocopy? I mean, I cant force my users to copy files with robocopy from
now on.... grml...

Thanks for help!