Booting of the capture kernel takes a very long time using vPMEM namespaces as a dump target for kdump/fadump

Solution Unverified - Updated

Environment

  • Red Hat Enterprise Linux 8.2

Issue

Booting of the capture kernel takes a very long time

Resolution

This is a known issue. It is being tracked under Red Hat Private Bugzilla 1792125. Open a service ticket for more information on it through the Red Hat Customer Portal.

It is advisable to not use vPMEM namespaces as a dump target for kdump/fadump.

Root Cause

During capture kernel (kdump/fadump) boot, the papr_scm module is forced to un-map and re-map the memory backed by vPMEM and to re-add it to its linear map.

This requires expensive HCalls to the PHYP hypervisor and total time taken is proportional to the size of vPMEM available to the PowerVM LPAR. Net effect of this is that it can slow down the boot of the capture kernel considerably.

Diagnostic Steps

If a user still prefers to use vPMEM memory as a dump target for kdump or fadump, the "papr_scm.ko" module must be explicitly included in the initrd. To achieve this create the file /etc/dracut.conf.d/99-pmem-orkaround.conf with the below content:

      add_drivers+="nd_pmem nd_btt libnvdimm papr_scm"

And then rebuild the initrd:

      # touch /etc/kdump.conf
      # systemctl restart kdump.service
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