Slow cifs write speeds with 4.13 kernel

[Bernd Schultze]

I have two ubuntu mate devices, a PC with ubuntu mate 17.10 and a thinkpad with 16.04.3. Both use samba shares from a synology diskstation accessed via cifs (in fstab). I always had about 110 MB/s write speed to the diskstation. Always when there is a 4.13 kernel involved, the write speeds drop to 20 MB/s.
What I tried: Different smb client protocol versions (vers parameter 1.0, 2.0, 2.1, 3.0). There are minor write speed differences of a few MB between the versions.

What I found out about the kernels and their impact on the write speeds
17.10 4.13.0-16 slow speeds
17.10 4.13.0-25 slow speeds
16.04.3 4.13.0-26 slow speeds
16.04.3 4.10.0-28 normal speeds > 100MB/s (due to the 4.13 default behaviour changes I have to explicitly specify the smb version to 3.0)

In a windows vm on the PC I get the normal speeds. An ubuntu server with an older kernel also gets the full speed.

A scp connection from the PC to another device also shows the normal speed.

In all scenarios, read speeds are fine (> 100MB/s).

I would gladly provide further informations or do some more testing but I am running out of ideas.

[Bernd Schultze (berndschultze)]

I tried to reproduce the problem in a VM today with a mate 17.10 with 4.13.0.-25 kernel and the normal fast write speeds. So I retried it on my PC (mate 17.10 4.3.0-25) and I had good write speeds again. I don't know what the problem was but I set the bug to invalid as it is working correctly and I can not reproduce the problem.

[Bernd Schultze (berndschultze)]

The word "occured" is missing at the end of the first sentence.
First good write speeds in the VM, then again on my PC.