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------- Comment From thorsten.diehl@xxxxxxxxxx 2016-04-15 09:39 EDT-------
well, here my complete proposed wording:

Depending on the number of available devices the amount of memory
specified by the "crashkernel" kernel parameter in /etc/zipl.conf may
not be sufficient. One can either increase it further, or limit the
amount of devices visible to the kernel, and thus lower the requirements
for the "crashkernel" setting.

To ignore devices you can run the "cio_ignore" tool to generate an
appropriate stanza to ignore all devices, except the currently active
/in-use.

$ sudo cio_ignore -u -k
cio_ignore=all,!c000-c002,!e000,!e100

You can simply add the "cio_ignore" kernel parameter with the stanza
from above to KDUMP_CMDLINE_APPEND in /etc/default/kdump-tools and
activate the setting with "systemctl restart kdump-tools.service".

As an alternative which blacklists the channel devices also for the
production system, you can add the generated stanza to the boot
parameters in /etc/zipl.conf. To activate the setting call "zipl -V" and
then reboot the system.

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https://bugs.launchpad.net/bugs/1564475

Title:
  128M is not enough for kdump on s390 LPARs

Status in Ubuntu on IBM z Systems:
  Fix Released
Status in makedumpfile package in Ubuntu:
  Invalid
Status in s390-tools package in Ubuntu:
  Fix Released
Status in zipl-installer package in Ubuntu:
  Fix Released
Status in makedumpfile source package in Xenial:
  Invalid
Status in s390-tools source package in Xenial:
  Fix Released
Status in zipl-installer source package in Xenial:
  Fix Released

Bug description:
  == Comment: #0 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 10:59:26 ==
  With the current Ubuntu default setting "crashkernel=128M" kdump on LPARs crashes with out-of-memory (see attachment "dmesg_lpar_out_of_mem_128M.txt").

  On z/VM guests 128M seems to be sufficient.

  One reason on our test LPAR is that a lot of devices are attached (see
  attachment "lscss_lpar.txt") which are not required for kdump but
  consume a lot of memory because the s390 CIO layer allocates data
  structures in the kernel for those devices.

  We can disable the devices by using the "cio_ignore=" kernel parameter
  in "/etc/default/kdump-tools". For example, on our LPAR that uses DASD
  0.0.e934 for /var/crash, we added the following line to disable the
  devices:

  KDUMP_CMDLINE_APPEND="irqpoll maxcpus=1
  cio_ignore=all,!condev,!0.0.e934"

  For more information on the "cio_ignore=" kernel parameter see:
  https://github.com/torvalds/linux/blob/master/Documentation/s390/CommonIO

  Even with "cio_ignore=" we still get out-of-memory with
  "crashkernel=128M".

  With "crashkernel=196M" and "cio_ignore=" we are able to create a dump
  on our LPAR. We currently do not know why kdump with "cio_ignore=" on
  LPAR consumes more memory than on z/VM guests.

  == Comment: #1 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 11:03:15 ==
  Kernel messages of kdump out-of-memory crash on LPAR with many devices without cio_ignore parameter and 128M crashkernel memory.

  == Comment: #2 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 11:04:10 ==
  Output of lscss showing all attached (not online) devices on the LPAR.

  == Comment: #3 - Michael Holzheu <michael.holzheu@xxxxxxxxxx> - 2016-03-31 11:07:35 ==
  To solve this issue our recommendation is:

  1) Increase "crashkernel=" default to 196M on Ubuntu for s390.

  2) Document that KDUMP_CMDLINE_APPEND with "cio_ignore=" can be used
  to decrease memory consumption for kdump on systems with many devices
  that are not required for kdump.

  The most user friendly solution would be to automatically determine
  the required kdump devices and set the correct "cio_ignore=" kernel
  parameter. But this is not trivial, because it can be difficult to
  find out the required devices for stacked setups like LVM or for
  network dump.

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