devices: disk: Set discard=unmap by default for some cases

This recommendation came from an internal discussion. The cases are

* For block storage. This means guest requests are passed through
  to the host device, which seems a more reasonable default than
  ignoring them

* For sparse disk images we will create. discard=unmap helps preserve
  the sparseness of the disk image. If a user requests non-sparse, they
  are likely more concerned with performance than saving disk space,
  so we leave the default as is. We limit this to disk images we will
  create, since that's the easiest case to check, and it's less clear
  if we should change the behavior here for an arbitrary existing
  disk image.

Signed-off-by: Cole Robinson <crobinso@redhat.com>

tests/data/cli/compare/virt-install-aarch64-kvm-gic.xml

@@ -22,7 +22,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-aarch64</emulator>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.qcow2"/>
       <target dev="vda" bus="virtio"/>
     </disk>

tests/data/cli/compare/virt-install-boot-uefi-notpm.xml

@@ -28,7 +28,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.qcow2"/>
       <target dev="sda" bus="sata"/>
     </disk>

tests/data/cli/compare/virt-install-graphics-usb-disable.xml

@@ -31,7 +31,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/fedora26.qcow2"/>
       <target dev="vda" bus="virtio"/>
     </disk>

tests/data/cli/compare/virt-install-kvm-session-defaults.xml

@@ -33,7 +33,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/tmp/.local/share/libvirt/images/disk.qcow2"/>
       <target dev="vda" bus="virtio"/>
     </disk>
@@ -105,7 +105,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/tmp/.local/share/libvirt/images/disk.qcow2"/>
       <target dev="vda" bus="virtio"/>
     </disk>

tests/data/cli/compare/virt-install-many-devices.xml

@@ -39,7 +39,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type="block" device="disk">
-      <driver name="qemu" type="raw" cache="writeback" io="threads" iothread="3" queues="8"/>
+      <driver name="qemu" type="raw" cache="writeback" discard="unmap" io="threads" iothread="3" queues="8"/>
       <source dev="/dev/default-pool/UPPER"/>
       <target dev="vda" bus="virtio"/>
       <serial>WD-WMAP9A966149</serial>
@@ -48,15 +48,15 @@
       <boot order="2"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2" discard="unmap" detect_zeroes="unmap" error_policy="enospace"/>
+      <driver name="qemu" type="qcow2" detect_zeroes="unmap" error_policy="enospace"/>
       <source file="/dev/default-pool/new1.img"/>
       <target dev="vdb" bus="virtio"/>
       <address type="drive" controller="0" target="2" unit="0"/>
       <readonly/>
     </disk>
     <disk type="file" device="cdrom">
+      <driver discard="ignore" copy_on_read="on"/>
       <target dev="sdaa" bus="sata"/>
-      <driver copy_on_read="on"/>
       <iotune>
         <read_bytes_sec>1</read_bytes_sec>
         <write_bytes_sec>5</write_bytes_sec>
@@ -67,7 +67,7 @@
       <readonly/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.qcow2"/>
       <target dev="vdc" bus="virtio"/>
     </disk>
@@ -186,12 +186,12 @@
       <target dev="fda" bus="fdc"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/dev/default-pool/new2.img"/>
       <target dev="sdad" bus="usb" removable="on"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/tmp/brand-new.img"/>
       <target dev="vdn" bus="virtio"/>
       <boot order="10" loadparm="5"/>
@@ -213,13 +213,13 @@
       <target dev="vdo" bus="virtio"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="raw"/>
+      <driver name="qemu" type="raw" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.img"/>
       <target dev="vdp" bus="virtio"/>
       <transient shareBacking="yes"/>
     </disk>
     <disk type="block" device="disk">
-      <driver name="qemu" type="raw" cache="none" io="native"/>
+      <driver name="qemu" type="raw" cache="none" discard="unmap" io="native"/>
       <source dev="/dev/disk-pool/disk"/>
       <target dev="vdq" bus="virtio"/>
     </disk>
@@ -229,7 +229,7 @@
       <target dev="vdr" bus="virtio"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/fooroot.img"/>
       <target dev="vds" bus="virtio"/>
       <transient/>

tests/data/cli/compare/virt-install-openbsd-defaults.xml

@@ -31,7 +31,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-x86_64</emulator>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.qcow2"/>
       <target dev="hda" bus="ide"/>
     </disk>

tests/data/cli/compare/virt-install-osinfo-url-with-disk.xml

@@ -38,7 +38,7 @@
       <target dev="vda" bus="virtio"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.qcow2"/>
       <target dev="vdb" bus="virtio"/>
     </disk>
@@ -96,7 +96,7 @@
       <target dev="vda" bus="virtio"/>
     </disk>
     <disk type="file" device="disk">
-      <driver name="qemu" type="qcow2"/>
+      <driver name="qemu" type="qcow2" discard="unmap"/>
       <source file="/var/lib/libvirt/images/disk.qcow2"/>
       <target dev="vdb" bus="virtio"/>
     </disk>

tests/test_cli.py

@@ -605,8 +605,8 @@ vcpus.vcpu1.id=2,vcpus.vcpu1.enabled=yes
 --iothreads 5
 
 --disk type=block,source.dev=/dev/default-pool/UPPER,cache=writeback,io=threads,perms=sh,serial=WD-WMAP9A966149,wwn=123456789abcdefa,boot_order=2,driver.iothread=3,driver.queues=8
---disk source.file=%(NEWIMG1)s,sparse=false,size=.001,perms=ro,error_policy=enospace,discard=unmap,detect_zeroes=unmap,address.type=drive,address.controller=0,address.target=2,address.unit=0
---disk device=cdrom,bus=sata,read_bytes_sec=1,read_iops_sec=2,write_bytes_sec=5,write_iops_sec=6,driver.copy_on_read=on,geometry.cyls=16383,geometry.heads=16,geometry.secs=63,geometry.trans=lba
+--disk source.file=%(NEWIMG1)s,sparse=false,size=.001,perms=ro,error_policy=enospace,detect_zeroes=unmap,address.type=drive,address.controller=0,address.target=2,address.unit=0
+--disk device=cdrom,bus=sata,read_bytes_sec=1,read_iops_sec=2,write_bytes_sec=5,write_iops_sec=6,driver.copy_on_read=on,geometry.cyls=16383,geometry.heads=16,geometry.secs=63,geometry.trans=lba,discard=ignore
 --disk size=1
 --disk /iscsi-pool/diskvol1,total_bytes_sec=10,total_iops_sec=20,bus=scsi,device=lun,sgio=unfiltered,rawio=yes
 --disk /dev/default-pool/iso-vol,seclabel.model=dac,seclabel1.model=selinux,seclabel1.relabel=no,seclabel0.label=foo,bar,baz,iotune.read_bytes_sec=1,iotune.read_iops_sec=2,iotune.write_bytes_sec=5,iotune.write_iops_sec=6

virtinst/devices/disk.py

@@ -995,14 +995,27 @@ class DeviceDisk(Device):
         if self.is_cdrom():
             self.read_only = True
 
+        discard_unmap = False
+        if (self.conn.is_qemu() and
+            self.is_disk() and
+            self._storage_backend.will_create_storage() and
+            self._storage_backend.get_vol_install() and
+            self._storage_backend.get_vol_install().allocation == 0):
+            discard_unmap = True
+
         if (self.conn.is_qemu() and
             self.is_disk() and
             self.type == self.TYPE_BLOCK):
+            discard_unmap = True
             if not self.driver_cache:
                 self.driver_cache = self.CACHE_MODE_NONE
             if not self.driver_io:
                 self.driver_io = self.IO_MODE_NATIVE
 
+        if discard_unmap:
+            if not self.driver_discard:
+                self.driver_discard = "unmap"
+
         if not self.target:
             used_targets = [d.target for d in guest.devices.disk if d.target]
             self.generate_target(used_targets)