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[Daniel Berrange <1616782@xxxxxxxxxxxxxxxxxx>]

This is not a bug - there are two completely independent settings here

 - nova.conf  hw_disk_discard is applicable to compute node local ephemeral storage
 - cinder 'discard' property is applicable to cinder volume storage

Using hw_disk_discard when configuring cinder volumes is not
appropropriate.

** Changed in: nova
       Status: In Progress => Invalid

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https://bugs.launchpad.net/bugs/1616782

Title:
  driver_discard='unmap' is written without considering the
  configuration

Status in OpenStack Compute (nova):
  Invalid

Bug description:
  The original code is as follows：

          if data.get('discard', False) is True:
              min_qemu = nova.virt.libvirt.driver.MIN_QEMU_DISCARD_VERSION
              if self.connection._host.has_min_version(
                      hv_ver=min_qemu,
                      hv_type=host.HV_DRIVER_QEMU):
                  conf.driver_discard = 'unmap'
              else:
                  global SHOULD_LOG_DISCARD_WARNING
                  if SHOULD_LOG_DISCARD_WARNING:
                      SHOULD_LOG_DISCARD_WARNING = False
                      LOG.warning(_LW('Unable to attach %(type)s volume '
                                      '%(serial)s with discard enabled: qemu '
                                      '%(qemu)s or later is required.'),
                                  {
                          'qemu': min_qemu,
                          'serial': conf.serial,
                          'type': connection_info['driver_volume_type']
                      })

  https://github.com/openstack/nova/blob/master/nova/virt/libvirt/volume/volume.py#L96

  No consideration is given to the case when the configuration file is
  not configured as hw_disk_discard=unmap

  Only when the backend report can support discard, is to write the
  unmap in XML

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