[Samba] Can't copy large files to Windows with SMB2/3 on 10G network

[Jeremy Allison]

On Sun, Sep 16, 2018 at 09:53:59AM +0200, MI via samba wrote:
> We have a 10G network with a mix of Windows 10 and MacOS 10.12 machines, and
> a CentOS 7 machine.
> 
> Copying large files "fast" from the CentOS machine to Windows, using SMB2 or
> SMB3 fails almost every time, at the end of the copy.
> - With SMB1, it seems to work, but is much slower.
> - Copying to a smb share on a Mac seems to work, but is also much slower.
> - Using a normal Gigabit network card instead of 10G, it always works.
> 
> By "large files" I mean 50 to 100 GB.
> By "fast" I mean over 300 MBytes per second (usually 500 to 700 MB/s.) as
> reported by various monitors on the Linux client or the windows servers.
> By "slower" I mean under 150 MB/s.
> 
> I tried both smbclient and mount.cifs:
> - smbclient [no -m option] is OK and slow (uses default SMB1/NT1?)
> - mount.cifs -o vers=1.0 is OK and slow
> - smbclient -m SMB2 (or -m SMB3) fails

The above is the interesting case, as this is much
easier to debug (in user space) than the kernel
client.

Can you give more details on *exactly* HOW smbclient -mSMB3
fails ?