[Bug 1434996] [NEW] TCP Zero Window on client NFS writes

[Ian]

Public bug reported:

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.

** Affects: nfs-utils (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: nfs performance tcp window zero

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https://bugs.launchpad.net/bugs/1434996

Title:
  TCP Zero Window on client NFS writes

Status in nfs-utils package in Ubuntu:
  New

Bug description:
  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.

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