TCP Zero Window on client NFS writes

[Ian]

When clients write to their NFS shares on 14.04 the transfers are very slow (often a few KB/s) and the server sends TCP Zero Window packets. The transfer then stops for anywhere up to 40 seconds. The transfer will then start again before pausing shortly after. This cycle continues until the transfer finally completes.

The server is not under any pressure and I can't see how it can justify sending a TCP Zero Window Packet.

Reading from the same NFS shares are pefect and data is always served at 9.5MB/s +

When writing to the same folders on the server using a different protocol such as sftp, the write performance is as expected, which is around 6MB/s. No transfer interruptions or problems experienced.

No problems reported in the server logs.

When monitoring the server during a transfer (using dstat) I can see that network activity stops or dips when the server writes to disk.

Spec / details
----------------------
Description: Ubuntu 14.04.1 LTS
Release: 14.04
nfs-kernel-server:
  Installed: 1:1.2.8-6ubuntu1.1
  Candidate: 1:1.2.8-6ubuntu1.1

Issue occurs using NFSv3 and NFSv4

I expect to be able to transfer data to the server at 6MB/s + without any delays, interrupts or problems.

[Ian (ian-waugh87)]

Issue is especally prevelant with the "sync" option enabled on NFS shares. Often the data transfer is stalled for upwards of 50 seconds.
With async option enabled on the shares the write performace is improved, but TCP zero window packets are still issued by the server. Even though the server is otherwise idle, apart from the initiated file transfer.