If Cinder reports discard=true capability then add
discard=unmap to disk config setting on attached volume.
To have this actually provide unmap/trim support the instance must have
been configured with a controller+bus type that can support it. As of
now the only one known not to work is virtio-blk. In the case where we
detect a configuration that will not work a warning will be logged, but
the attachment will continue with the default behavior.
An example of how to get a controller and bus to support this is to set
the following Glance properties on an image:
hw_scsi_model=virtio-scsi
hw_disk_bus=scsi
This feature also has some requirements for the libvirt and qemu
versions being used. There is a check to ensure that libvirt is at least
1.0.6 and qemu is at least 1.6.0. If the minimum versions are not met
a warning will be logged, but the attachment will continue with the
default behavior.
Change-Id: Icd2d20bb6906d9106df34a970b3d81b17bc3ec07
Implements: bp cinder-backend-report-discard
6bc074587a