Libvirt-Enable suppport for discard option for disk device

Support setting discard option for instance's disk.
If discard option is enabled, unused/freed blocks can be return back to the
storage device.

blueprint libvirt-disk-discard-option

Change-Id: Idcf7b35c93422d0d13c3ffb33aca3c5eca4f3c94

specs/juno/libvirt-disk-discard-option.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+======================================================
+Libvirt-Enable support discard option for disk device
+======================================================
+
+https://blueprints.launchpad.net/nova/+spec/libvirt-disk-discard-option
+
+Most SCSI devices have supported UNMAP command which is used to return unused
+or freed blocks back to the storage. And SSD drive have a similar command
+called "Trim" command.
+
+This blueprint aims to support setting discard option for instance's disk.
+If discard option is enabled, unused/freed blocks can be return back to the
+storage device.
+
+Qemu1.5 has supported setting discard option for the raw disk. In Qemu1.6,
+qcow2 file supported discard option too.
+
+Cinder support is out of scope for this spec. This spec only covers disks
+managed by nova.
+
+Problem description
+===================
+
+Thin provision volume
+---------------------
+When we write data to a thin provision volume, storage device will allocate
+blocks to it and it will grow.
+But without discard option supported, the blocks will not be freed to the
+storage device even if we deleted the data in the volume. The result is the
+thin volume can grow, but can't shrink.
+With discard option supported, when user deleted the data in the volume,
+the blocks will be freed to the storage device. The volume will shrink.
+
+It's useful for both ephemeral volume and cinder volume.
+
+SSD backed volume
+--------------------
+Freeing the unused blocks to the SSD storage is useful to improving the
+performance and prolonging the lifetime of SSD.
+
+Proposed change
+===============
+
+Add support for the deployer to specify the discard option in the nova.conf by
+"hw_disk_discard".
+
+There are two available values for "hw_disk_discard" now::
+
+  "unmap" : Discard requests("trim" or "unmap") are passed to the filesystem.
+  "ignore": Discard requests("trim" or "unmap") are ignored and aren't passed
+  to the filesystem.
+
+For example::
+
+  hw_disk_discard=unmap    #enable discard
+  hw_disk_discard=ignore   #disable discard, by default
+
+For an instance running on a host which has the discard option in nova.conf,
+nova will produce the XML with a discard option when the nova managed disk
+is attached.
+
+Alternatives
+------------
+None
+
+Data model impact
+-----------------
+None
+
+REST API impact
+---------------
+None
+
+Security impact
+---------------
+None
+
+Notifications impact
+--------------------
+None
+
+Other end user impact
+---------------------
+None
+
+Performance Impact
+------------------
+Discard option maybe cause performance degradation and fragmentation.
+But for the storage based on SSD, discard option is good for the performance.
+
+The users have control over this behaviour in the guest os if they don't want
+it. They can use the mount parameter or the command tools to control the
+discard behaviour.
+
+Other deployer impact
+---------------------
+Initially, only the libvirt driver will support this function, and
+only with qemu/kvm as the hypervisor.
+
+A serious consideration is needed before enabling discard option.
+With discard option enabled, the freed blocks of the thin provision volume
+will be return to the storage and can be reused. But it also maybe cause
+performance degradation and fragments.
+So it's reasonable to enable the discard option only when you use the thin
+provision volume and the storage are UNMAP/TRIM-capable. For example the SSD,
+the disk-arrays or other distributed storage which supports the UNMAP/TRIM.
+
+Developer impact
+----------------
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+boh.ricky
+
+Work Items
+----------
+
+* Libvirt driver will create a discard option for a disk device which the
+  instance flavor has discard option.
+
+Dependencies
+============
+Libvirt(1.0.6) Qemu1.5(raw format) Qemu1.6(qcow2 format)
+
+Testing
+=======
+None
+
+Documentation Impact
+====================
+
+The document should be modified to reflect this new feature.
+
+References
+==========
+None