[Bug 1571761] Re: zfs-import-cache.service slows boot by 60 seconds

[Martin Pitt]

There is some angles of attack here:

└─zfs-import-cache.service @1min 786ms +272ms
  └─systemd-udev-settle.service @494ms +1min 275ms

This is a case of "you asked for it, you got it", really. udev-settle is
a workaround for old software that wasn't written with hotplugging in
mind. It's 2016, and we really should avoid introducing new dependencies
to udev-settle, as it's conceptually broken and can't work.  This
totally does not work for any hotplugged device, or a device which just
gets detected by the kernel late, or which needs to be unwrapped from
building an LVM or cryptsetup device, etc.

I think it would be much more elegant to instead create udev rules (or
systemd units) which load the ZFS cache as soon as a ZFS block device is
detected. Would that be possible/reasonable? I don't know anything about
what import-cache does, but I hope/suppose it has some way of doing this
on a per-device basis?

    └─systemd-udev-trigger.service @415ms +55ms

This is cloud-init's rules for blocking udev rules until network devices
get detected and cloud-init's naming rules get applied. This is a hairy
topic and Scott and I already discussed this at length. I think there's
a better solution here, but I doubt we'll completely turn this around by
the release. This just illustrates one of the problems of the "waiting
for all hardware just to single out a particular device" approach from
above -- this waits on stuff which is completely unrelated to zfs.

** No longer affects: systemd (Ubuntu)

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https://bugs.launchpad.net/bugs/1571761

Title:
  zfs-import-cache.service slows boot by 60 seconds

Status in cloud-init package in Ubuntu:
  Confirmed
Status in zfs-linux package in Ubuntu:
  Confirmed

Bug description:
  Fresh uvt-kvm guest, then
  $ sudo apt-get install zfsutils-linux
  $ sudo reboot

  The reboot will show on console waiting for tasks, then after ~ 60 seconds it continues boot.
  Logging in shows:

  $ sudo systemd-analyze critical-chain zfs-mount.service
  The time after the unit is active or started is printed after the "@" character.
  The time the unit takes to start is printed after the "+" character.

  zfs-mount.service +81ms
  └─zfs-import-cache.service @1min 786ms +272ms
    └─systemd-udev-settle.service @494ms +1min 275ms
      └─systemd-udev-trigger.service @415ms +55ms
        └─systemd-udevd-control.socket @260ms
          └─-.mount @124ms
            └─system.slice @129ms
              └─-.slice @124ms

  Seems possibly related / or some discussion at
  https://github.com/zfsonlinux/zfs/issues/2368

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: zfsutils-linux 0.6.5.6-0ubuntu8
  ProcVersionSignature: User Name 4.4.0-18.34-generic 4.4.6
  Uname: Linux 4.4.0-18-generic x86_64
  NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
  ApportVersion: 2.20.1-0ubuntu1
  Architecture: amd64
  Date: Mon Apr 18 16:42:35 2016
  ProcEnviron:
   TERM=screen.xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: zfs-linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  modified.conffile..etc.sudoers.d.zfs: [deleted]

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