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[Stéphane Graber <stgraber@xxxxxxxxxxxx>]

** No longer affects: lxd (Ubuntu Xenial)

** No longer affects: lxd (Ubuntu Zesty)

** No longer affects: lxd (Ubuntu Artful)

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https://bugs.launchpad.net/bugs/1567557

Title:
  Performance degradation of "zfs clone"

Status in Native ZFS for Linux:
  New
Status in zfs-linux package in Ubuntu:
  Fix Released
Status in zfs-linux source package in Xenial:
  Fix Committed
Status in zfs-linux source package in Zesty:
  Fix Committed
Status in zfs-linux source package in Artful:
  Fix Released

Bug description:
  [SRU Justification]

  Creating tens of hundreds of clones can be prohibitively slow. The
  underlying mechanism to gather clone information is using a 16K buffer
  which limits performance.  Also, the initial assumption is to pass in
  zero sized buffer to   the underlying ioctl() to get an idea of the
  size of the buffer required to fetch information back to userspace.
  If we bump the initial buffer to a larger size then we reduce the need
  for two ioctl calls which improves performance.

  [Fix]
  Bump initial buffer size from 16K to 256K

  [Regression Potential]
  This is minimal as this is just a tweak in the initial buffer size and larger sizes are handled correctly by ZFS since they are normally used on the second ioctl() call once we have established the size of the buffer required from the first ioctl() call. Larger initial buffers just remove the need for the initial size estimation for most cases where the number of clones is less than ~5000.  There is a risk that a larger buffer size could lead to a ENOMEM issue when allocating the buffer, but the size of buffer used is still trivial for modern large 64 bit servers running ZFS.

  [Test case]
  Create 4000 clones. With the fix this takes 35-40% less time than without the fix. See the example test.sh script as an example of how to create this many clones.


  
  ------------------------------------------

  I've been running some scale tests for LXD and what I've noticed is
  that "zfs clone" gets slower and slower as the zfs filesystem is
  getting busier.

  It feels like "zfs clone" requires some kind of pool-wide lock or
  something and so needs for all operations to complete before it can
  clone a new filesystem.

  A basic LXD scale test with btrfs vs zfs shows what I mean, see below
  for the reports.

  The test is run on a completely dedicated physical server with the
  pool on a dedicated SSD, the exact same machine and SSD was used for
  the btrfs test.

  The zfs filesystem is configured with those settings:
   - relatime=on
   - sync=disabled
   - xattr=sa

  So it shouldn't be related to pending sync() calls...

  The workload in this case is ultimately 1024 containers running busybox as their init system and udhcpc grabbing an IP.
  The problem gets significantly worse if spawning busier containers, say a full Ubuntu system.

  === zfs ===
  root@edfu:~# /home/ubuntu/lxd-benchmark spawn --count=1024 --image=images:alpine/edge/amd64 --privileged=true
  Test environment:
    Server backend: lxd
    Server version: 2.0.0.rc8
    Kernel: Linux
    Kernel architecture: x86_64
    Kernel version: 4.4.0-16-generic
    Storage backend: zfs
    Storage version: 5
    Container backend: lxc
    Container version: 2.0.0.rc15

  Test variables:
    Container count: 1024
    Container mode: privileged
    Image: images:alpine/edge/amd64
    Batches: 128
    Batch size: 8
    Remainder: 0

  [Apr  3 06:42:51.170] Importing image into local store: 64192037277800298d8c19473c055868e0288b039349b1c6579971fe99fdbac7
  [Apr  3 06:42:52.657] Starting the test
  [Apr  3 06:42:53.994] Started 8 containers in 1.336s
  [Apr  3 06:42:55.521] Started 16 containers in 2.864s
  [Apr  3 06:42:58.632] Started 32 containers in 5.975s
  [Apr  3 06:43:05.399] Started 64 containers in 12.742s
  [Apr  3 06:43:20.343] Started 128 containers in 27.686s
  [Apr  3 06:43:57.269] Started 256 containers in 64.612s
  [Apr  3 06:46:09.112] Started 512 containers in 196.455s
  [Apr  3 06:58:19.309] Started 1024 containers in 926.652s
  [Apr  3 06:58:19.309] Test completed in 926.652s

  === btrfs ===
  Test environment:
    Server backend: lxd
    Server version: 2.0.0.rc8
    Kernel: Linux
    Kernel architecture: x86_64
    Kernel version: 4.4.0-16-generic
    Storage backend: btrfs
    Storage version: 4.4
    Container backend: lxc
    Container version: 2.0.0.rc15

  Test variables:
    Container count: 1024
    Container mode: privileged
    Image: images:alpine/edge/amd64
    Batches: 128
    Batch size: 8
    Remainder: 0

  [Apr  3 07:42:12.053] Importing image into local store: 64192037277800298d8c19473c055868e0288b039349b1c6579971fe99fdbac7
  [Apr  3 07:42:13.351] Starting the test
  [Apr  3 07:42:14.793] Started 8 containers in 1.442s
  [Apr  3 07:42:16.495] Started 16 containers in 3.144s
  [Apr  3 07:42:19.881] Started 32 containers in 6.530s
  [Apr  3 07:42:26.798] Started 64 containers in 13.447s
  [Apr  3 07:42:42.048] Started 128 containers in 28.697s
  [Apr  3 07:43:13.210] Started 256 containers in 59.859s
  [Apr  3 07:44:26.238] Started 512 containers in 132.887s
  [Apr  3 07:47:30.708] Started 1024 containers in 317.357s
  [Apr  3 07:47:30.708] Test completed in 317.357s

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