I have a trouble with my windows 2008 R2 KVM guest. It write speed to disk extremely slow: 
But on another VM (windows 10) on this KVM host write speed is ok:
# cat /etc/system-release
CentOS Linux release 7.1.1503 (Core)
# virsh -V
Virsh command line tool of libvirt 1.2.8
See web site at http://libvirt.org/
Compiled with support for:
Hypervisors: QEMU/KVM LXC ESX Test
Networking: Remote Network Bridging Interface netcf Nwfilter VirtualPort
Storage: Dir Disk Filesystem SCSI Multipath iSCSI LVM Gluster
Miscellaneous: Daemon Nodedev SELinux Secrets Debug DTrace Readline Modular
# qemu-system-x86_64 -version
QEMU emulator version 2.0.0, Copyright (c) 2003-2008 Fabrice Bellard
Slow VM XML configuration:
<domain type='kvm' id='24'>
<name>SLOW-VM</name>
<uuid>405f55de-ee46-7d1a-4915-a82b617f5706</uuid>
<memory unit='KiB'>8388608</memory>
<currentMemory unit='KiB'>8388608</currentMemory>
<vcpu placement='static'>2</vcpu>
<resource>
<partition>/machine</partition>
</resource>
<os>
<type arch='x86_64' machine='pc-i440fx-rhel7.0.0'>hvm</type>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/libexec/qemu-kvm</emulator>
<disk type='block' device='disk'>
<driver name='qemu' type='raw'/>
<source dev='/dev/vg/slow-vm'/>
<backingStore/>
<target dev='vda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>
<disk type='block' device='floppy'>
<driver name='qemu' type='raw'/>
<source dev='/dev/vg/slow-vm-2'/>
<backingStore/>
<target dev='fda' bus='fdc'/>
<alias name='fdc0-0-0'/>
<address type='drive' controller='0' bus='0' target='0' unit='0'/>
</disk>
<controller type='fdc' index='0'>
<alias name='fdc0'/>
</controller>
<controller type='usb' index='0'>
<alias name='usb0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
</controller>
<controller type='pci' index='0' model='pci-root'>
<alias name='pci.0'/>
</controller>
<serial type='pty'>
<source path='/dev/pts/1'/>
<target port='0'/>
<alias name='serial0'/>
</serial>
<console type='pty' tty='/dev/pts/1'>
<source path='/dev/pts/1'/>
<target type='serial' port='0'/>
<alias name='serial0'/>
</console>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<graphics type='vnc' port='5901' autoport='yes' listen='127.0.0.1' keymap='en-us'>
<listen type='address' address='127.0.0.1'/>
</graphics>
<video>
<model type='cirrus' vram='16384' heads='1'/>
<alias name='video0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<alias name='balloon0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>
Another VM XML configuration:
<domain type='kvm' id='21'>
<name>vm</name>
<uuid>e48f03ad-aced-452b-9e85-904ee5be61cb</uuid>
<memory unit='KiB'>1048576</memory>
<currentMemory unit='KiB'>1048576</currentMemory>
<vcpu placement='static'>2</vcpu>
<resource>
<partition>/machine</partition>
</resource>
<os>
<type arch='x86_64' machine='pc-i440fx-1.5'>hvm</type>
<boot dev='cdrom'/>
<boot dev='hd'/>
</os>
<features>
<acpi/>
<apic/>
<pae/>
</features>
<cpu mode='custom' match='exact'>
<model fallback='allow'>cpu64-rhel6</model>
<feature policy='require' name='nx'/>
</cpu>
<clock offset='localtime'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>restart</on_crash>
<devices>
<emulator>/usr/bin/qemu-system-x86_64</emulator>
<disk type='block' device='disk'>
<driver name='qemu' type='raw' cache='none'/>
<source dev='/dev/vg/vm'/>
<backingStore/>
<target dev='vda' bus='virtio'/>
<alias name='virtio-disk0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
</disk>
<disk type='file' device='cdrom'>
<driver name='qemu' type='raw'/>
<backingStore/>
<target dev='hdd' bus='ide'/>
<readonly/>
<alias name='ide0-1-0'/>
<address type='drive' controller='0' bus='1' target='0' unit='0'/>
</disk>
<controller type='usb' index='0'>
<alias name='usb0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
</controller>
<controller type='ide' index='0'>
<alias name='ide0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
</controller>
<controller type='pci' index='0' model='pci-root'>
<alias name='pci.0'/>
</controller>
<serial type='pty'>
<source path='/dev/pts/4'/>
<target port='0'/>
<alias name='serial0'/>
</serial>
<console type='pty' tty='/dev/pts/4'>
<source path='/dev/pts/4'/>
<target type='serial' port='0'/>
<alias name='serial0'/>
</console>
<input type='tablet' bus='usb'>
<alias name='input0'/>
</input>
<input type='mouse' bus='ps2'/>
<input type='keyboard' bus='ps2'/>
<graphics type='vnc' port='5903' autoport='yes' listen='127.0.0.1' keymap='en-us'>
<listen type='address' address='127.0.0.1'/>
</graphics>
<video>
<model type='cirrus' vram='16384' heads='1'/>
<alias name='video0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
</video>
<memballoon model='virtio'>
<alias name='balloon0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
</memballoon>
</devices>
</domain>
I try to use cache = 'none' and io = 'native' – without result.
I try to change emulator and machine type on the slow VM – without result.
lsblk:
sdd 8:48 0 931.5G 0 disk
|-sdd1 8:49 0 500M 0 part
| `-md0 9:0 0 499.7M 0 raid1 /boot
`-sdd2 8:50 0 931G 0 part
`-md1 9:1 0 1.8T 0 raid10
|-vg-slowvm 253:0 0 180G 0 lvm
|-vg-vm 253:13 0 24G 0 lvm
Where is misconfiguration?
Best Answer
The problem was on the virtio driver on the slow Windows guest. I have change it to the bus=scsi. + I set on the KVM host profile: virtual-host via adm-tune.