[Bug 1722481] Re: systemd should sync disks, before killing all processes on shutdown

[Dimitri John Ledkov]

As seen in autopkgtests, reboots are working correctly with filesystems
synced to disk on shutdown, without regressions in shutdown / reboot.
Thus this is sufficient to verify that the disks are still synced on
shutdown.

** Tags removed: verification-needed verification-needed-xenial
** Tags added: verification-done verification-done-xenial

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https://bugs.launchpad.net/bugs/1722481

Title:
  systemd should sync disks, before killing all processes on shutdown

Status in systemd package in Ubuntu:
  Fix Released
Status in systemd source package in Xenial:
  Fix Committed

Bug description:
  [Impact]

   * systemd-shutdown binary is executed to complete system shutdown. In
  xenial, it calls sync() after killing all processes, and not before
  killing them. This means that processes are racing the SIGKILL/TERM
  timeouts to sync all IO to disk. To mitigate this race condition, and
  make the shutdown process more reliable the sync() should also happen
  before process killing spree starts.

  [Fix]
  Backport upstream commit https://github.com/systemd/systemd/commit/2e79d1828a8da9b3af1b052297e3617905ec94f3

  
  [Test Case]

   * Make sure systems still shuts down, including e.g. root on raid.

  [Regression Potential]

   * Shutdowns may appear to be slower due to two sync() calls instead of one
   * However total shutdown time should not be impacted much, as there really should not be much additional IO caused by killing all processes.

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